[S08] Earthquake physics

10:45 AM - 11:00 AM JST(1:45 AM - 2:00 AM UTC)

[S08-07] How Short Are Dominant Seismic Wavelengths in Reality?

*Koji UENISHI¹ (1. The University of Tokyo)

By analyzing dynamic rupture behavior of natural and artificial structures, we have been trying to show the existence of shorter seismic waves with audible higher frequencies over 16 Hz that are verbally reported for near-field earthquakes repeatedly (e.g. Uenishi and Sakurai, Earthquake Eng. Struct. Dyn., 2000; Uenishi, ASME J. Appl. Mech., 2012; Physics Today, 2021; Proc. Struct. Int., 2024). Apparently, such higher frequencies cannot be correctly recorded by ordinary seismographs and therefore, they have been seldomly treated in seismogram-based studies. However, nowadays, digital devices such as drive and handy video recorders including smartphones and tablets are widely available, and they are undoubtedly enhancing our "visibility" of seismic shaking, rupture propagation and their influence on the surroundings. Visual observations and studies with video recorders having 30 fps (frames per second) may enable us to directly "watch" previously unrecognized rupture and wave phenomena up to at least 30 Hz, which is far above the current "limit" of seismological analyses, i.e. some 10 Hz. For instance, on the occasion of the 2024 Noto earthquake, besides a dynamically winding road recorded in Suzu City, a drive recorder located at a parking lot in Wakura, Nanao City, clearly shows vertically jumping pavement and surfacing dynamic rupture as well as massive leakage from a water storage tank and swinging overhead lines supported by utility poles. Another drive recorder entering an intersection in Anamizu Town also shows violently swinging overhead lines. If the resonance of the overhead lines in Anamizu Town is evaluated analytically with typical mechanical properties, the associated wavelength and frequency are 16 m and 17 Hz for the fundamental mode and 8 m and 34 Hz for the second mode, respectively. Incidentally, according to a photograph of the distorted railroad track between Nanao and Wakuraonsen Stations, dominant seismic wavelength may be found to be roughly 12 m. That means, when an anti-plane shear wave with a typical speed of some 400 m/s is assumed to have caused the distortion of the track, the dominant frequency may be 33-34 Hz. This value is close to the second mode of resonance of the overhead lines in Anamizu Town. Note also that the fundamental resonance 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 station (Uenishi and Sakurai, Earthquake Eng. Struct. Dyn., 2000; although not mentioned in the paper, the second resonance frequency is some 374 Hz). Thus, both dynamic rupture-based and handy video-based analyses can indicate the existence of shorter or higher-frequency dominant seismic waves, and the latter show a possibility of visually supported seismology or "visual seismology" in addition to the well-established and strong seismogram-based seismology. If the number of videos showing dynamic movements on the ground surface, etc., caused by near-field earthquakes increases, the existence of shorter seismic waves may become more evident.

Acknowledgements: The research has been financially supported by the Japan Society for the Promotion of Science (JSPS) through the "KAKENHI: Grant-in-Aid for Scientific Research (C)" Program under grant number 23K04021.