PSI - Issue 54

Koji Uenishi et al. / Procedia Structural Integrity 54 (2024) 67–74 Uenishi / Structural Integrity Procedia 00 (2023) 000–000

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7

a

b

60

6

NS

NS

0

0

]

]

-60

-6

2

2

60

6

EW

EW

0

0

-60

-6

Abrupt vertical shock

60

6

Acceleration [cm/s

Acceleration [cm/s

TD

TD

0

0

-60

-6

0 102030405060

0 102030405060

Time [s]

Time [s]

Fig. 5. (a) Seismograms recorded at the site ISKH01 Suzu (latitude 37.5266  N, longitude 137.2844  E) of the KiK-net of NIED for an earthquake

that occurred at 9:23 am JST of March 23, 2022. The latitude and longitude of the epicenter of the quake are 37.515  N and 137.298  E, with a

focal depth of 14 km and M j = 4.3. The maximum accelerations in the NS, EW and UD directions at ISKH01 with the epicentral distance of only

2 km are 51.824, 73.312 and 89.405 cm/s 2

. (b) Seismological records at the same TKY007 Shinjuku in Tokyo for an earthquake that occurred at

7:10 pm JST of July 9, 2022. This earthquake’s epicentral latitude and longitude are 35.595  N and 139.640  E, with a focal depth of 33 km and M j

= 3.6. The maximum accelerations in the NS, EW and UD directions at TKY007 with the epicentral distance of 14 km are 2.535, 2.635 and 6.079

2 (original data provided by NIED (2022)).

cm/s

seismographs so far, it cannot be concluded that there exist no vertical shocks nor seismic waves with very high

frequencies (Uenishi, 2021). For example, after Kumamoto, another “audible” shaking, a “thump” and a jolt from

below, has been reported for the September 6, 2018 Hokkaido Eastern Iburi earthquake with a focal depth of 37 km

and M j = 6.7 in the daily newspaper Kobe Shimbun issued on September 13, 2018.

In addition, more recently obtained seismograms recorded at the site ISKH01 Suzu of the KiK-net of NIED for

one of the earthquake swarms suggest that if the epicenter is close and the amplitude of the shaking is relatively

small, seismological records may seem more realistic (Fig. 5(a)). The small earthquake occurred on March 23, 2022,

j = 4.3. The maximum vertical shaking at ISKH01 with the epicentral distance of

with a focal depth of 14 km and M

just 2 km is stronger than the horizontal one, but there is a rather strong peak of the horizontal shaking 1.87 seconds

after the maximum vertical shaking. The records in Tokyo at the same TKY007 Shinjuku as in Fig. 1 might be more

interesting. Figure 5(b) indicates that even at the identical site, if the epicenter is close, the vertical shaking can

become dominant. Here, the dataset is for an earthquake that occurred on July 9, 2022, with a focal depth of 33 km

and M j = 3.6. Although experienced in the same room, my personal feeling was totally different from the previous

earthquakes shown in Fig. 1, and so were the seismograms at TKY007 with the epicentral distance of 14 km. The

abrupt vertical shaking at the very beginning is stronger than any other following horizontal and vertical shaking,

and this sudden vertical shaking seems to be consistent with what we really feel when an earthquake occurs just

underneath. Indeed, in the velocity response spectra of this earthquake, the vertical component is dominant for

almost all frequencies below 10 Hz, even in a lower frequency range. This is a clear evidence of the existence of an

earthquake where the vertical shaking is stronger than the horizontal one for all structures having eigenfrequencies

below 10 Hz. However, the difficulty in handling larger amplitudes and frequencies over 10 Hz, especially over 16

Hz, still remains unsolved.

5. Conclusions

By monitoring and collecting seismological records and analyzing dynamic performance of structures step by

step, we have been trying to verify the existence of the vertical shock or vertical shaking with audible higher

frequencies verbally reported for near-field earthquakes occurring just underneath. The photographs taken in Kobe