Abstract
Infrasonic signals measured before an earthquake carry information about the size and development speed of the source fracture, the stress at the fracture site and the elastic properties of the geologic medium. The infrasonic signal has a stable time scale, and compared with other precursors, infrasound has a unique sensitivity to earthquake disasters. However, to date, there has been no relevant theoretical research on the mechanism of infrasonic anomalies, and information on the development of fracture sources cannot be obtained from these characteristics, which makes the application of this anomaly in earthquake prediction challenging. In this study, we obtained the characteristics of short-term and impending infrasonic anomalies based on the infrasound data of more than 100 strong earthquakes. With a range of elastic medium models with a large number of fractures, we completed the theoretical simulation of the formation process of infrasonic precursors during the formation of the main fractures, analyzed the physical evolution of acoustic signals when cracks are generated, and quantitatively described the stages of large fracture formation caused by the initiation and propagation of seismic cracks. Specifically, this study revealed the causes of various and complex forms of infrasonic precursors near the critical point and the causes of the time- and space-dependent characteristics of these precursors, such as a noticeable attenuation of the pulse number, a low frequency and a large amplitude, which verified the effectiveness of infrasonic anomalies as strong earthquake precursors.
Highlights
The interpretation of acoustic precursor characteristics is the basis of the understanding of the main failure mechanism of earthquakes or rock fractures
We summarized the anomalous infrasonic features before a strong earthquake as follows
Based on the analysis of the rock fracture process and mechanical performance under an axial force, Li et al [29] found that the trend of the variation in acoustic emission (AE) from rock samples was basically consistent with the observed stages of the failure process
Summary
The interpretation of acoustic precursor characteristics is the basis of the understanding of the main failure mechanism of earthquakes or rock fractures. Two models that interpret the development of precursor phenomena before an earthquake have been proposed: the expansion-dispersion model and the avalanche-type unsteady fracture formation model These models differ greatly, they share the same presumption regarding the changes in the mechanical characteristics of the medium during the loading and expansion process. Based on the abovementioned analysis, apart from their characteristics of sudden and rapid changes, the infrasonic anomalies are complex and diverse in morphology, and the time at which the maximum anomalous amplitude appears corresponds to the point of highest energy (as shown in Figs 3, 5 and 7); the dominant frequency is low and appears at approximately 10−3 Hz (as shown in Figs 4, 6 and 8). The sound wave propagation velocity and absorption coefficient can be expressed as follows:
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