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As you may know, Trypophobia is a phobia whose main symptom is sensitivity to relatively small or porous things such as lotus pods, soap bubbles, frog eggs and beehives (from perplexity.ai, similar to the recently popular ChatGPT). For some people, dense things are more scary, but unexpectedly, earthquakes also have "dense phobias." Where should we start? Let's start with seismic attributes.

Color Bubble (Source: Sohu) earthquake, as far as we understand it, is ground motion. The tremor of the earth's crust conveys to the ground, if we do not feel it, it is an earthquake without earthquake sensation. If the sky falls apart, then the magnitude of the earthquake is even higher. Earthquake magnitude aside, we always receive quick reports of earthquakes, which roughly contain the following key information: magnitude, time of occurrence, location, depth and epicenter distance (your distance from the epicenter of the ground earthquake).

Quick report screenshot of the earthquake, have you ever thought about how the information came from? Those who have a certain understanding or have seen around the environment must know the seismic stations. This is the fixed earthquake monitoring system. The seismograph network formed by the seismological stations all over the country is the seismograph network-- the intensive Seismological observation Network (Dense Observation Network), which is the heavenly eye for earthquake monitoring. Therefore, the relevant information of earthquake occurrence can be located by using seismic network and some technical means (earthquake location multi-station method, etc.). However, for regional earthquakes, the number of seismic stations is relatively small, which leads to some limitations. Then why not build more? Considering the actual cost and utility, if the country is not rich enough to allocate funds for scientific research at will, there will not be too many fixed seismic stations. Therefore, compared with the fixed seismic station, the mobile seismic station (that is, node seismograph) also reflects its advantages.

Node seismograph (SmartSolo) node seismograph is widely used in regional scientific research by geophysicists because it is easy to carry. On the other hand, different layout methods will have different effects on different scientific problems. In recent years, with the development of computer and storage, the calculation of data processing has been greatly improved. at this time, the "dense array", the super-dense seismic array (Dense Seismic Array), came into being. The dense array benefits from its intensive layout and has a good ability to solve regional geophysical problems. So why can dense array effectively solve regional geophysical problems? This is because intensive observation can obtain more observation data, combined with subsequent processing and analysis, more effective information can be obtained. For example, in the aspect of earthquake location, the dense array can obtain more earthquake travel time information, which provides more constraints for its accurate location.

ESeis onshore node seismic instruments reached the international leading level and realized industrialization (Science Network) as early as 2011, researchers from relevant colleges and universities in the United States set up a dense array with a spacing of about 100m at long Beach (Long Beach) in Los Angeles. 5200 seismographs were monitored continuously with 500Hz sampling for 6 months, covering a work area of 10km * 7km. Another dense array was set up in 2012, with a number of 2500, with the Newport-Inglewood fault zone below it. Although it was an observation experiment ten years ago, the relevant work around this experiment has been published one after another until now, which is the characteristics of scientific research work. The development of different technologies and methods in different periods can have a better understanding of the old data.

In Los Angeles long Beach, two dense arrays, Blue frame I and Green frame II (Lei et al., 2022), were used by Inbal and other researchers to study the seismicity beneath the array in 2016. How to study activity? The basis of the research is earthquake location. We know that the characteristic of dense array is the similarity of data between various seismographs. How are they similar? Let's look at the specific data. The following picture is the seismic data after deep learning and denoising (about depth learning, we will talk about it in more detail in the following tweets). The waveform data of each seismograph are arranged according to the epicenter distance, and the horizontal and longitudinal coordinates are time and epicenter distance respectively. We can see an obvious propagation process of the seismic signal, and the origin of the earthquake can be accurately located by using this information.

The denoised seismic signal (Lei et al., 2022) is identified by the seismic signal, and then located, the result of earthquake location in the observation period can be obtained, and then its activity can be understood. The results of this "SCIENCE" article in 2016 show that there is seismicity in the upper mantle of the transition fault. Generally speaking, the seismicity of the transition fault on land is basically limited to the upper crust, while the seismicity of the Newport-Inglewood fault zone also exists in the upper mantle, which is not consistent with the previous cognition and belongs to a new discovery. Interestingly, however, an article published in SCIENCE ADVANCES in 2022 showed that the Newport-Inglewood fault zone had no seismicity in the upper mantle.

No matter who is right and who is wrong, only one thing we get has arrived, the process of scientific cognition is tortuous and rising! Those who are interested in the corresponding specific content and technical details can read the original text.

Earthquake location depth result map (Inbal et al., 2016) dense array technology is widely used in regional research in recent years, and it also makes a qualitative leap in geophysics. This is why earthquakes also have "dense phobias"!

References:

[1] Inbal, A., Ampuero, J. P., & Clayton, R. W. (2016). Localized seismic deformation in the upper mantle revealed by dense seismic arrays. Science, 354 (6308), 88-92.

[2] Yang, L., Liu, X., Zhu, W., Zhao, L.A., & Beroza, G. C. (2022). Toward improved urban earthquake monitoring through deep-learning-based noise suppression. Science advances, 8 (15), eabl3564.

This article is from the official account of Wechat: stone popular Science Studio (ID:Dr__Stone), author: Xingyu

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