Shulou(Shulou.com)11/24 Report--

On the morning of August 6, I was suddenly frightened by the folks in my circle of friends. Shandong, where the weather was favorable, was hit by an earthquake. According to the official determination of the China Seismological Network, an earthquake of magnitude 5.5 occurred at 02:33 on August 6 in Pingyuan County, Dezhou City, Shandong Province, with a focal depth of 10 km and an epicenter at 37.16 degrees north latitude and 116.34 degrees east longitude. A magnitude 3.0 earthquake occurred 03:02 on August 6 in Pingyuan County, Dezhou City, Shandong Province, with a focal depth of 9 km and an epicenter at 37.16 degrees north latitude and 116.34 degrees east longitude.

Why did Shandong, where there is a grandma of Mount Tai, turn over its car? Why did the earthquake have a low magnitude but a strong feeling? Why is such a small magnitude so destructive? Is the earthquake in the early morning the main shock, or is it just a foreshock? Today we are going to have a good chat about the biggest earthquake in Shandong in the past ten years.

1. Shandong is not stable. In fact, the frequent occurrence of earthquakes in Shandong is not a quiet place. According to the historical data of the China Seismological Network in the past five years, Shandong has experienced an average of more than 2,000 earthquakes a year, but 90% of these earthquakes are not felt by people. For example, in July this year, Shandong network recorded 16 natural earthquakes in Shandong and its coastal areas, including 7 earthquakes of magnitude less than 1, 7 earthquakes of magnitude 1.0-1.9, 2 earthquakes of magnitude 2.0-2.9, 0 earthquakes of magnitude 3.0 or above, and the maximum earthquake of magnitude 2.9 in the Yellow Sea at 17:00 on the 17th. The epicenter of the earthquake occurred within 200 kilometers of Pingyuan County, Dezhou City, Shandong Province, a total of 8 earthquakes of magnitude 3 or above occurred in the past five years.

Epicenter distribution map of earthquakes with M ≥ 3 in Shandong since 2023 (from Shandong earthquake Express) Shandong Province is intertwined with the Tan-Lu fault zone, like a wandering dragon. The Tan-Lu fault zone has been regarded as a key earthquake risk monitoring area by the State Seismological Bureau since 1990. it was formed by the collision between the North China plate and the South China plate. The confluence of these two plates creates the natural barrier between the south and the north of China-the Qinling-Dabie Mountain Line, which is the boundary between the north and the south and carries the historical weight of many geography and culture. The large and small earthquakes in Shandong area are more or less related to the geological activity of the Tan-Lu fault zone, but is this earthquake also because of the Tan-Lu fault zone?

In fact, according to the map, there is a certain distance between the Tan-Lu fault zone and the epicenter of this earthquake, so the possibility of this earthquake caused by the Tan-Lu fault zone is relatively small, and it should be an earthquake caused by the geological movement within the North China block. At the same time, the North China plate tectonics has also created several groups of southwest-to-northeast-trending fault zones, all of which have certain seismicity in history. For example, Heze, south of the Liaocheng-Lankao fault zone, which induced this earthquake, occurred successively on August 1, 1937, an earthquake of M = 7.0 and M = 6.3, and an earthquake of M = 6.0 in November 1983). At the same time, we also note that the epicenter of this earthquake does not seem to be located in any major seismic zone, which makes the cause and mechanism of the earthquake more complex, which is worthy of our in-depth study. We need to further observe and study the seismic data in order to better understand the causes of seismicity in Pingyuan County, Dezhou.

In a word, no matter from the existing seismic data statistics or geographical location analysis, the location of Shandong Province is not calm, but a place with hidden dangers. We still need to remain vigilant and take corresponding preventive measures to maintain a high degree of vigilance in the face of severe geological disasters.

2. Why is the earthquake felt obviously but with a low magnitude? The earthquake was obviously felt because of the shallow depth of the source. The shallowness of the focal depth means that the seismic energy is released closer to the surface, so the seismic wave is more likely to spread to the surface, resulting in an obvious earthquake feeling. At the same time, Dezhou City is located in the vast and flat Haihe Plain, with an average elevation of only 24 meters near the epicenter, covered with sediment deposits of the Yellow River and Haihe River with a depth of 800 meters. The structure of this part of the sedimentary layer is weak, and the propagation velocity of seismic wave in it is significantly slower than that of the hard rock at the bottom, and the refraction and reflection at the bottom interface increase the amplitude of ground motion, so it has a certain magnifying effect on the ground motion. Under the same focal scale and propagation path, the existence of the local sedimentary soil layer makes the ground motion on the surface greater, so the earthquake feeling will be more obvious.

In addition, if there is no earthquake in a region for a long time, the underground medium is relatively complete and continuous, and the seismic wave attenuates slowly in the process of propagation, so that the seismic wave is easier to transmit and the range of influence will be wider. Even in relatively distant places, the propagation of seismic waves may be felt and tremors may be felt.

In addition, the background is quiet in the middle of the night, the noise of the city gradually fades away, there are few vehicles on the road, and people's activities are also gradually reduced, which makes the surrounding environment quieter and people's spirit more alert. In such a quiet atmosphere, the faint sound and vibration produced by the earthquake can be easily detected. Even an earthquake with a smaller magnitude may feel more obvious in the stillness of the night.

3. When we only look at the magnitude without considering the intensity, we may misunderstand the destructive power of the earthquake. Intensity and magnitude are two different concepts in seismology and often confuse people.

Magnitude is an indicator of the amount of energy released by an earthquake. It is usually expressed by magnitude scales, such as the Richter scale (Richter Scale) or the body wave magnitude (Moment Magnitude Scale,Mw). The magnitude is a logarithmic scale, and for each additional unit of magnitude, the energy released by the source of the earthquake increases by about 32 times. Therefore, an increase of one magnitude means that the increase in earthquake energy release is huge. Magnitude is often used to evaluate the size and intensity of earthquakes, but the magnitude does not directly reflect the impact of earthquakes on human society.

Intensity is an index used to describe the feeling and influence of earthquakes on the earth's surface. Intensity scales are usually used. At present, there are many kinds of intensity meters in the world, and the 12-degree scale is more common. In addition, there are octave meters (Japan and Taiwan Province) and ten-degree meters.

The seismic intensity is derived from and applied to the Rossi-Forrell seismic intensity scale of 10 degrees, which was revised by Italian volcanologist Giuseppe McGhali in 1883 and 1902. Later, it was revised many times by a number of geographers, seismologists and physicists to become today's revised Magali seismic intensity scale (Modified Mercalli intensity scale). The intensity of the Macquarie earthquake ranged from imperceptible to completely damaged by 1 to 12 degrees. Damage can only be caused by 5 degrees or more, and the intensity will vary from place to place. The revised McGhali earthquake intensity is now used in the United States, South Korea, Hong Kong and other places. Revise the Maigali seismic intensity scale and set out the MSK intensity adopted in India, the former Soviet Union, Israel and other places, and then derive the EMS intensity adopted in Europe and the Chinese mainland intensity standard (CMMI).

Intensity is a subjective assessment that determines the intensity of an earthquake at a particular location by investigating and recording the impact of an earthquake on people, buildings, and the environment. The intensity is not directly related to the energy of the source, and the magnitude of the same magnitude may produce different intensity values in different locations.

The magnitude of this earthquake is relatively high, but the intensity is not necessarily very strong. According to the China Seismological Network, corresponding to the intensity model, the earthquake may have an intensity of 7 degrees in several villages within 10 kilometers of the epicenter-which will have a more obvious destructive impact; on the other hand, the surrounding plain, Wucheng and some urban areas of Texas will have an intensity of about 6 degrees, with slight-mild destructive effects and strong tremors. Farther away from the epicenter, there is usually no destructive effect, so there is no need to worry.

Intensity level (source: Feng Rui. Interesting Seismology (10): intensity, related to people's livelihood [J]. Advances in Seismological Science, 2019, (10): 54-64. Doi: 10.3969 / j.issn.0253-4975.2019.10.009)

4. Is the earthquake in the early morning the main shock, or is it just a foreshock? Foreshock and mainshock are two kinds of earthquake events in earthquake sequence, and there is a certain relationship between them. The foreshock is a small earthquake that occurs before the main shock, and the main shock is the largest earthquake event in the earthquake sequence. Foreshocks are precursors or precursors of the main shock, which indicate that stress in the earth's crust is accumulating and may lead to larger seismic events, that is, the main shock. The main shock is an earthquake with the strongest energy release and the largest magnitude, which usually causes the most obvious earthquake feeling and damage.

Explanation of the definition of foreshock, mainshock and aftershock in earthquake sequence (source: Abdelnaby A. Multiple earthquake effects on degrading reinforced concrete structures [J]. 2012.)

Foreshocks are characterized by a small magnitude and a small range of seismic waves, which may not be easy to detect. However, foreshocks are important for predicting possible large earthquakes because they indicate that stress in the earth's crust is accumulating and may lead to larger earthquake events. In fact, everyone's concern about the Texas earthquake is not unreasonable, but this question is answered when the fault research is not yet in place.

To judge whether it is a foreshock or not generally depends on seismic monitoring data and seismological analysis. The earthquake monitoring organization will continuously monitor the seismic activity in the earth's crust and record the earthquake magnitude, focal depth, epicenter location and other information. Therefore, it is not easy to judge whether it is a foreshock or not.

Earthquake prediction is still a complex scientific field, and there is not an accurate and reliable method to predict the occurrence time and specific scale of earthquakes. Although foreshocks may herald the possibility of a mainshock, not all foreshocks cause mainshocks, and not all mainshocks have foreshocks.

This article is from the official account of Wechat: stone Science Popularization Studio (ID:Dr__Stone), by Shao Jingying

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