Standing Waves in Epicentral Areas of Earthquakes

Often, a sharp increase or decrease in seismic wave level cannot be explained by existing seismic microzoning methods, especially since at magnitudes of 8–9, the amplitude does not increase on loose soils; on the contrary, they are somewhat lower than on rocky soils. At high amplitudes, seismic wave attenuation is no longer determined by the properties of the medium, but by the vibration level. The absorption decrement sharply increases, and the resonance properties of soils subside until they disappear completely. Moreover, the near zone is located above the focal zone, where not only absorption but also energy release takes place. The article discusses three processes that can explain anomalous variations in seismic wave amplitudes. First, the amplitude may increase with distance from the fault, since the fault may occur not in the area of maximum deformation, but at the area of minimum strength of the medium, i.e., in the area of a previously formed fault. However, such a model poorly agrees with the fact that zones of anomalous amplitudes are often arranged symmetrically with respect to the fault. The second reason is that, naturally, the fault plane can contain no energy; the fault only allows the release of accumulated deformation energy in the surrounding medium. Consequently, the energy of seismic waves propagating from the fault surface will increase until elements of the medium release more energy than they absorb. This model is confirmed by empirical data. Acceleration amplitudes during earthquakes of different magnitudes scale well with a shift along the distance axis and not along the amplitude axis, as is commonly believed. Many researchers have shown that, based on empirical data, the maximum level of acceleration in the focal region does not depend on magnitude, but is determined by the type of movement along the fault. It has been shown empirically that the level of acceleration at magnitudes of 9–11 is the same. Seismic intensity above 9 is no longer determined by the acceleration amplitude, but by residual deformations, i.e., changes in the relief. The two described processes do not explain the phenomenon of alternating zones of increasing and decreasing amplitude under the same soil conditions observed in earthquake focal areas. The authors propose an idea for discussion that explains all of the above phenomena: the occurrence of standing waves in the focal region. The article presents information on the occurrence of anomalous amplitude variations, which can be explained by the theory of standing waves. Zones of anomalous increase and decrease in amplitudes can be interpreted as zones of nodes and antinodes of standing waves. Empirical equations are proposed for distances at which zones of nodes and antinodes may appear.