PSI - Issue 77

Koji Uenishi et al. / Procedia Structural Integrity 77 (2026) 183–189 Uenishi / Structural Integrity Procedia 00 (2026) 000–000

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3. Asymmetric wave confining effect of curved planes of weakness In the previous chapter, it is found the wave field induced by an energy source moving along a curved plane of weakness is indeed asymmetric, and for all Mach number regimes the induced waves seem to be more strongly confined and therefore larger influence like severer vibration is expected on the convex side of the interface. This seems to be true when we look at the distribution of the seismic intensity related to the Noto Peninsula, Japan, near field earthquake on 1 January 2024 (Japan Meteorological Agency (JMA) magnitude 7.6, Fig. 3(a)). In Noto, the main seismic rupture (usually equivalent to mode-II in-plane shear fracture), initiated at a depth of 16 km, propagated bilaterally. To the southwest, it propagated initially along rather straight geological planes of weakness (fault segments) and then it changed the propagation directions to the south along largely curved fault segments. The largest seismic intensity (JMA intensity 7) was recorded on the convex side of the ruptured fault system at Hashiride, Monzenmachi in Wajima City (epicentral distance of some 50 km) and Kano, Shika Town (65 km) where the induced waves may be much more easily and strongly confined to some limited areas close to the fault segments. Simplified numerical model analyses by Uenishi (2025) may support this idea of confined wave energy on the convex side. Note that at places closer to the epicenter but located along the straight fault segments, at Otanimachi in Suzu City (epicentral distance 8 km) and Fugeshimachi in Wajima City (35 km), the JMA seismic intensity was smaller, 6+. During the passage of such confined wave energy on the convex side, high frequencies may be experienced. Although analyses of dynamic rupture behavior of natural and human-made structures do reinforce the idea of the possible presence of shorter seismic waves with audible higher frequencies over 16 Hz that have been verbally reported for near-field earthquakes repeatedly (e.g. Uenishi and Sakurai (2000), Uenishi (2024)) and there does exist reliable verbal evidence given by professionals like “horizontal winding of the Yodo River railway tunnel with apparent amplitude of 1 m and wavelength of 40-50 m” (Kawata (1996)), such higher frequencies cannot be properly recorded by ordinary seismographs and therefore, they are hardly handled in seismogram-based studies. But these days, digital devices such as drive and compact video recorders including smartphones and tablets are quite commonly available, and they are unquestionably enhancing our “visibility” of seismic shaking, rupture development and their effect on the surroundings. “Visual” observations and studies with video recorders with 30 fps (frames per second) may enable us to directly view so far unrecognized rupture and wave phenomena up to at least 30 Hz, which is by far higher than the current “ceiling” of seismological analyses, approximately 10 Hz. For example, on 1 January 2024, in addition to a dynamically winding road in Suzu City, a video by a drive recorder at a parking lot in Wakura, Nanao City (near Kakiyoshimachi in the identical city), clearly shows vertical jumping of the pavement and surfacing of dynamic rupture as well as massive leakage from a water storage tank and swinging of the overhead transmission lines supported by utility poles. Another drive recorder entering an intersection in Anamizu Town similarly shows violent swinging of the overhead lines. If the overhead lines are regarded as strings and their resonance frequency f is simply evaluated analytically, then f = c T / λ with the transverse wave speed along the overhead lines = � / and the wavelength λ of some vibration mode (see Fig. 3(b) and (c), with tension and linear density of the overhead lines T and ρ , distance between utility poles L ). If typical values ( T = 6 kN, ρ = 0.125 kg/m, L = 6.5 m) are employed for the intersection in Anamizu Town, the associated wavelength and frequency are λ = 2 L = 13 m and f = 17 Hz for the fundamental vibration mode and λ = L = 6.5 m and f = 34 Hz for the second mode, respectively. Incidentally, a photograph of the distorted railroad track between Nanao and Wakuraonsen Stations suggests that dominant seismic wavelength λ may be roughly 12 m. That is, when an anti-plane shear wave with a typical speed c S of about 400 m/s for engineering bedrock is considered to have caused the distortion of the track, the dominant frequency may be f = c S / λ ≈ 400/12 = 33-34 Hz. This value is very close to the second mode of vibration of the overhead lines in Anamizu Town. Note also that the fundamental frequency of the Anamizu overhead lines, 17 Hz, is approximately equal to that of the central columns at the Daikai Station in Kobe estimated for another near-field earthquake in 1995 through the collapse of the underground station (Uenishi and Sakurai (2000); although not mentioned in the original paper, the second resonance frequency is some 374 Hz). Thus, both dynamic structural failure-based and compact video-based analyses can show the possible presence of shorter or higher-frequency dominant seismic waves, and the latter analyses indicate a new possibility of visually supported seismology or “visual seismology” in addition to the well-established and strong seismogram-based seismology.