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When we speak of earthquakes, some regard them literally as shaking or vibrations in our living environments, either on the surface or in the ground, while others may be reminded of wave propagation in solid earth. In short, research in seismology has been developed to find more reasonable relations between the vibrations (records of seismographs) and “invisible” waves transmitted underground (and sometimes in the air). However, the words “vibration” and “wave” are often used confusedly, especially in engineering applications, and therefore we must learn the fundamental connections between them with educational books before we start discussing the details of the seismic fragility of our society, etc.
Late Professor Keiiti Aki (J Seism Soc Jpn, vol. 57 pp. 317–319, 2005, or his lecture note in 2003) indicated that there are essentially two groups formed in seismology or the study of the links between vibrations and waves. One is the “smooth earth club” where the deterministic approach...