Two scientific development trends of earthquakes (II)

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Seismology is a discipline based on observation and measurement data. With the changes in social needs and the development of disciplines, digitization has been achieved one by one with the capture, acquisition, recording, transmission, and interpretation of seismic signals. The field of traditional seismology has been studied. Changes are taking place. In view of its recent development, the following two trends are worth our attention:

1 Transition from seismic hazards to earthquake studies

Reducing earthquake disasters has always been an important driving force for the development of earthquake science. In the early days, the earthquake disaster was mainly caused by the collapse of building construction. Therefore, seismology and engineering were combined to develop seismic engineering. Its main purpose was to conduct seismic risk analysis and provide reasonable and economical engineering measures. The basis for the foundation.
In recent years, two facts have changed the trend of this kind of research.

First, with the continuous acceleration of the process of urbanization of the world’s population and the continued development of the world’s economy, the loss of social disasters caused by a destructive earthquake has not only become more and more serious, but it has also begun to break the long-established project losses represented by the destruction of buildings. The dominant earthquake disaster pattern, especially for densely populated and economically developed big cities, is likely to account for only a small part of engineering disasters in future earthquakes. In place of "engineering disasters" caused by earthquakes, the concept of "social disaster losses" caused by earthquakes has emerged. For a long time, the project loss caused by the destruction of houses and buildings has been the main part of the earthquake disaster losses, and it is also a focus of research in seismology. However, with the development of urbanization, the earthquake losses are increasingly not limited to Loss of engineering, business interruption, and social functions. Loss of information, such as loss of non-engineering, is an increasing proportion. The Great Hanshin Earthquake on January 17, 1995 caused more than 6,000 deaths and more than 100 billion U.S. dollars in damage. The loss of buildings and facilities caused the loss of more than 48 billion U.S. dollars. Since Osaka in the epicentral area is an important port of Japan, the economic losses caused by the disruption of traffic after the earthquake, economic slumps, and the interruption of import and export trade amounted to as much as 60 billion U.S. dollars, and caused serious psychosocial unrest and unemployment. And people’s ineffectiveness in government disaster relief caused a decline in trust in the government. It can be seen that in areas with developed economy and high level of urbanization, once a destructive earthquake occurs, it will cause a huge loss of society, and the loss will no longer be confined to simple engineering losses. In 1995, RMS (Risk Management Solution Inc., RMS) simulated the consequences of the repeated occurrence of the earthquake of the same size in the Kanto Earthquake of Japan in 1923. The results of the analysis surprisingly show that the comprehensive loss of earthquakes will reach USD 2,100 billion. The loss of buildings and internal facilities was only $100 billion, less than 50%. For most areas, engineering disaster losses are still the most important part of the earthquake disaster, but destructive earthquakes occur in areas with high degree of urbanization, economic development, or adjacent areas, and the proportion of engineering disasters will be significantly reduced. The resulting loss from social disasters will become more and more serious. With the rapid concentration of population to urban areas and the rapid development of social economy, this type of disaster with social loss as the main body will surely break the earthquake damage pattern dominated by long-term construction disasters represented by building damage. Therefore, it is necessary to put forward the concept of “social disaster” caused by earthquakes. It should refer to the comprehensive impact of an earthquake on human society, including engineering disasters such as building destruction, and non-engineering disasters such as commercial and traffic interruptions and follow-up. Long-term effects on the economy, society, and people's psychology.

Second, in recent years, the record of losses caused by earthquakes has been continuously updated in many countries. For example, in 1994, the Northridge earthquake in the United States broke the record of the loss of natural disasters in the United States; the loss caused by the Chichi earthquake in Taiwan also set a new record for earthquake losses in Taiwan; in 2001, the magnitude 7.7 earthquake in Gujarat, India was the record so far. One of the largest intra-plate earthquakes to arrive was one of the worst casualties in India’s history. The economic losses caused have also hit the Indian economy hard. Earthquake disasters are always accompanied by huge loss of life and property, the development of the global economy, and the creation of a large amount of social wealth, which has caused tremendous changes in the targets of earthquake disasters, especially in the vulnerability of modern society to earthquake disasters. Become more and more vulnerable. In the 20th century, with the passage of time, the loss caused by earthquake disasters has a growing trend. Cruel reality requires seismologists to study the earthquake itself more and study the seismic risk.

At present, research on mitigating earthquake disasters is changing from facing engineering disasters to facing social disasters. Research in this area has differentiated into two prominent development directions. One is that earthquake-prone countries such as Japan and China carry out large-scale seismic risk assessments in key economic regions and densely populated areas. For example, earthquake activity fault detection and surface deformation Analysis, site response analysis, etc.; another development direction is the close coordination of the government and seismologists to formulate emergency response plans for large earthquakes and build post-earthquake response systems and rescue systems.


2 Transition from deep structural studies to shallow crustal structures In recent years, observations of some seismic stations constructed on sedimentary layers have shown that seismic waves from different directions have different receiver functions. This has caused the seismological community to re-understand the characteristics of the shallow, especially sedimentary wave velocity. After a few years of field work in The SKIPPY Project, which began in Australia in 1993, B. Kennyt et al. found that the S-wave velocities exhibited by receiver functions at different azimuth angles are clearly discrete. This is just an example of a seismic station located in a thin area of ​​the sedimentary layer.
The P-wave velocity in the extremely shallow sediments obtained by the laboratory measurement is only 0.6-1.8 km/s, the S-wave velocity is 0.3-0.6 km/s, the wave speed ratio is 2 to 3, and even reaches 6, which is equal to the average mid-wave velocity It's a far cry from 1.7 or so. Although the thickness of the sediment layer is not large in the wave propagation path, the wave travel time in the sediment layer has a large impact on the overall travel time of the wave. It has been found that the results of the early internationally known deep-sensing surveys such as PANDA did not take into account the influence of shallow sediments, and the results needed to be reconsidered. This fact shocked the entire academic community. The "Los Angeles Regional Seismic Test" (LARSE) conducted in the thick sedimentary Los Angeles Basin in the United States also observed seismic waves of 1.75 km/s [1-2], which are commonly used to process earthquake locations and retrieve crustal velocity structures. The wave speed is about 6.0km/s.
The description of the characteristics of shallow seismic wave velocity changes requires detailed understanding of sediment thickness, lateral variation, and even seasonal variations. The modification of the existing crustal velocity model using the shallow sedimentary layer velocity has gradually become an important area in the study of the three-dimensional velocity model of the crust. The shallow velocity structure, ie, the refinement of the crustal velocity model over a depth of 1 km, has increasingly become a bottleneck in improving the inversion accuracy of three-dimensional crustal velocity.
In the north of China and the central part of the United States, the thickness of the sedimentary layer below some seismic stations is close to 1 km. For example, of the 107 digital seismic stations newly constructed and already put into operation in the “Capital Zone Earthquake Preparedness and Disaster Reduction Demonstration Zone” project, 58 stations use short-period pendulums in the underground, basically thought to be laid on bedrock; there are also 49 Stations use wideband and very wide frequency pendulum and are deployed on the surface of the sediment layer. The large attenuation of shallow velocities may directly affect the interpretation of the data recorded by seismic observation stations. If there is a breakthrough in the study of shallow sedimentary structures around 1 km, then some velocity structures recorded by seismic stations based on sedimentary layers may be recalculated and analyzed. Research on shallow sedimentary structures is increasingly Become a highlight of seismological research.

Concluding remarks With the continuous advancement of monitoring technology and the rapid accumulation of digitized data, seismology is focusing on the development of the plate boundary zone, the combination of depth and velocity structures, theoretical numerical simulation and seismic hazard research.

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