The Roles of Higher-Frequency Waves in “Unexpected” Problems of Earthquakes

Uenishi, Koji (The University of Tokyo)



Utilizing the theories of continuum mechanics, etc., the research work in seismology has been trying to establish more rational relations between (earth-)“quakes” (recorded “shaking” or “vibrations”) and “waves” that are initiated at seismic sources and propagated underground (and sometimes in the sea and the atmosphere). Like other dynamic fracture (rupture) phenomena in solids, however, an earthquake or rupture of solid Earth mostly occurs at a “concealed” place and even when dynamic rupture can be observed, its high-speed development cannot be followed by normal video recording devices. In addition, the multi-scale complexity of dynamic rupture in rocks and rock-like materials makes the comprehension of the fundamental mechanism of rupture development and relevant physical events much harder. Hence, in the normal treatment in seismology, the precise mechanical process around rupturing seismic sources is not straightforwardly taken into account and kinematic source parameters are mainly estimated from seismograms. Although valuable findings have been gained by the traditional methods, there exist not a few earthquake-induced structural failures that cannot be explained via the classical kinematic approach. Here, by handling some instances of “unexpected” earthquake-related failures, ranging from the collapse of underground facilities to the commencement of spontaneous crustal rupture at depth, we shall briefly mention the mechanics behind the “extraordinary” earthquake hazards. Rigorous yet plain dynamic analyses suggest that the “abnormal” failures are produced in fact for clear reasons. These obvious causes may remain unnoticed if conventional geophysical and geotechnical techniques are employed and only kinematic features of the lower-frequency components of seismic waves below 1 Hz are studied. Our analyses suggest that, in place of kinematics, dynamics considering the impact of higher-frequency waves over 1 Hz is required to systematically clarify the reasons for the “unusual” phenomena and reduce in the future the so-called “unexpected damage” due to earthquakes.

1. Introduction

The 2016 Kumamoto, Japan, earthquakes are a unique series of seismic events in the sense that a very strong quake (moment magnitude Mw 6.2), occurred at 9:26 pm on 14 April (local time), was immediately followed by the second but even stronger one (Mw 7.0) at 1:25 am on 16 April. Also from the shaking point of view, this series of earthquakes have provided clear and important evidence of the existence of higher-frequency up-down (vertical) oscillations in the epicentral region, which has been regarded as “out of question” especially in the engineering field. As normally supposed in traditional seismic analyses, also in the 2016 Kumamoto events, horizontal disturbances were dominant in a lower frequency range, but the seismograms obtained for the two shocks in Mashiki Town at the KMMH16 station (Fig. 1; epicentral distance 2 km) of the Japanese strong-motion seismograph network of the National Research Institute for Earth Science and Disaster Resilience (NIED) KiK-net indicate that the up-down shaking can become more influential than the horizontal one in a higher frequency range, e.g. over some 3.7 Hz (period shorter than 0.27 seconds) for the second shock (see Fig. 1b). As described below, the structural failure patterns found in the Kobe area on the occasion of the 1995 Hyogo-ken Nanbu, Japan, earthquake (Mw 6.9) also suggest the presence of higher-frequency vertical seismic waves, but regrettably, the sensitivity of the SMAC-MDU strong-motion accelerometers then normally installed fell considerably above 20 Hz and vibrations over 10 Hz were filtered out, and consequently, higher-frequencies were hardly detected (Uenishi, 2012). Now, at last, higher-frequencies can be recorded with seismographs in a way closer to reality.