The findings, according to the research leaders, PhD candidate Shahar Gvirtzman and Prof. Jay Feinberg from the Rakah Institute of Physics, are the beginning of the answer to the question regarding the existence of earthquakes of different intensities that originate from identical tectonic plates. The research was published in the prestigious journal Nature Physics
The contact area between any two bodies is not continuous and consists of discrete contacts that are pressed against each other, where the actual contact occurs. In the friction process, when one body tries to slide on the other body, the stressed contacts resist the movement, and in order for sliding to be possible, they have to break/separate. The moment of breaking contact and releasing the energy involved is also known as an earthquake, when the two bodies are tectonic plates that move relative to each other. A relevant example for Israel is of course the friction between the Arab plate and the Israeli-Chinese plate, along the Syrian-African fault, which extends along the Jordan Valley. Since the stressed contacts resist movement, the relative sliding between the plates is not a slow and continuous sliding, but occurs in sharp and rapid 'jumps'. With each such jump, the contacts break, the energy stored in them is released, and only then is sliding possible, after which the system 'gets stuck' again and begins to accumulate energy until the next earthquake.
In recent years, studies have shown that during an earthquake, the contacts do not all break 'at once', but in a gradual process in which they break one after the other, so that it is possible to define a kind of 'breaking wave' that moves rapidly along the fault, breaking the contacts and enabling the sliding. These waves were documented in detail, and today the researchers know a lot about the characteristics of these breaking waves, the speed of their progress, the conditions of their stopping, and more. On the other hand, much less is known about the 'nucleation' process of the break, i.e. how the initial break is formed from which the breaking waves develop. One of the reasons for this is that it is difficult to predict the location where the initial break will start and the conditions at that point, so it is difficult to measure the process well. A new study published in the prestigious journal Nature Physics, led by PhD student Shahar Gebirtzman and Prof. Jay Feinberg - both from the Rakeh Institute of Physics at the Hebrew University, attempted to examine the process of fracture initiation in an earthquake.
One of the understandings from which the researchers set out is that not only can we not predict when an earthquake will occur, we also cannot predict its size - even when it comes to the same tectonic plates that are in the same set of pressures, the resulting jumps can be of different sizes, and with a different amount of energy released each time. "In other words, although the global conditions that create the earthquakes in Israel do not change, small tremors that are not felt at all, as well as strong tremors that cause enormous damage, can occur in the same fault. The same uncertainty occurs in any movement of bodies in friction, and in fact earthquakes of different sizes are another manifestation of the problematic nature of the traditional physical description according to which the beginning of the movement of a body is determined by a characteristic 'coefficient of friction', which is a constant number that depends only on the type of material," explains Prof. Feinberg .
The new study showed that the source of the variation in the intensity of earthquakes is the initial nucleation process that created them. As part of the study, researchers in Prof. Feinberg's laboratory photographed in real time and in high resolution the area of contact between two bodies, on which pressures are applied, and recorded the fast breaking waves (earthquakes) moving between them, breaking the contacts and enabling the sliding. They have, in effect, produced miniature 'earthquakes' in the laboratory. Using a new technique that allows control over the location of the fracture initiation point, the researchers focused on the slow expansion of the initial fracture zone, and found that the fractured area grew very slowly until reaching a critical moment, where it grew beyond the minimum size necessary to create the fracture wave. At this moment, which is the 'opening shot' of the earthquake, the fracture accelerates and begins to move as a fast wave, in the manner known from previous studies. They also discovered that, unlike the later stage of the fast wave, where the progress does not depend on the exact location along the fracture, the stage of reaching the critical size also depends at the nucleation point itself. Characteristics such as the local pressures and the specific distribution of contacts in the nucleation area are the ones that determine, for example, the total time it will take for the system to reach that critical moment.
The same plates - different intensities of earthquakes
The findings, according to the researchers, are the beginning of the answer to the question regarding the existence of earthquakes of different intensities arising from identical tectonic plates. While the global characteristics (the composition of the plates, the rate of their movement, the stresses exerted on them) are the same, the characteristics of the earthquake depend largely on the local characteristics at the nucleation point, which changes each time. "This can be likened to a fire caused by extremely dry weather (the stored energy waiting to be released), but its characteristics and onset depend on the specific match that ignites it (the nucleation process), and on the timing when it arrives," Gvirtzman explains. "The detailed research documentation of the nucleation process is an opening to a better understanding of the processes that precede an earthquake, and can also help with a general understanding of the phenomenon of friction and related areas such as the formation of cracks in the material."
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