115【Memoirs of Construction Engineering Research Institute Vol.65(Research Overview)Nov.2023】

ONOTWOODISTINTTOTYPESOOFOSEISMITOSHAKINGOWEOMAYOEXPERIENTEO
Koji Uenishi
ＡＢＳＴＲＡＣＴ
We briefly discuss the two noticeable different types of seismic shaking: (1) vibration with rather long
duration that may be generated by interlocking earthquakes, seismic clusters or earthquake swarms; and(2) abrupt vertical shock experienced but probably seismologically incorrectly recorded near epicenters ofshallow earthquakes.

１． ＩＮＴＲＯＤＵＣＴＩＯＮ
There are still many unknowns in the seismic rupture processes. Examples include those related to two distinct typesof rather extreme seismic shaking1) with (1) very long and (2) impulsive duration. The former shaking (1) may arise
from interlocking earthquakes, seismic clusters as well as earthquake swarms, and for instance, the off the Coast of
Fukushima Prefecture earthquake at 23:36 JST on 16 March 2022 itself (JMA magnitude 7.4, epicentral latitude andlongitude of 37.697N and 141.622E, focal depth 57 km) has seismological records of long shaking of about twominutes. Considering its foreshock (JMA magnitude 6.1, epicentral latitude and longitude of 37.680N and 141.605E,
focal depth 57 km) that occurred just two minutes before, at 23:34 JST, the total duration for this (series of)earthquake(s) might be regarded as extraordinarily long four minutes. Although extraordinarily long, the shaking
generated by (interlocking) earthquakes seems to be relatively correctly, like what we feel by ourselves, recorded: The
vibration in the vertical direction is weaker than that in the horizontal direction in the far field as in the case of“ordinary” earthquakes.
In order to comprehend more thoroughly the generation mechanism of this and other seemingly complex rupture
processes associated with interlocking earthquakes, seismic clusters and earthquake swarms, we are conducting
laboratory two-dimensional experiments for simultaneously observing global, large-scale material behavior and local,
smaller-scale development of ruptures and waves in linear elastic brittle solids. The solid specimens involve sets ofdigitally prepared small-scale cracks that model large-scale geological fault planes, and they are subjected to externalquasi-static and dynamic impact loads. The specimen in Fig. 1, for example, has zones of damage (weak zones)
consisting of small-scale cracks in specific places. Intuitively, the pre-existing weak zones with a dip angle of 30
degrees are expected to accelerate rupture development, but the snapshots experimentally taken by a high-speed video
camera show that both upward quasi-static primary and downward dynamic secondary ruptures are “captured” in the
weak zones. Thus, pre-existing weak zones may decelerate and even arrest rupture development, and further
experiments suggest that local geometrical changes may indeed control the global and local (interlocking) rupturebehavior2).

２． MISSING DATA
The latter impulsive shaking (2) is abruptly experienced near epicenters of shallow earthquakes. The strong

116

vertical shock, such as the one felt onboard a ship at sea or seaquakes,
has been reported verbally for some earthquakes. However, thesudden and strong shock arising straight from beneath seems to havebeen incorrectly recorded in seismograms. Unfortunately, althoughexperienced repeatedly, the existence of the strong vertical shock canTensileloadingbe inferred only indirectly through several dynamic studies of theunique near-field seismic structural failures found on the surface andin the underground1). Therefore, we are currently trying to uncover
the physical properties of the shock by investigating recent
earthquake swarms around Noto Peninsula that draw much attention
and other small earthquakes that might attract much less attention likethe one at 19:10 JST on 9 July 2022 (JMA magnitude 3.6, epicentral
latitude and longitude of 35.595N and 139.640E, focal depth 33km). The sudden vertical shock recorded in the very beginning of the9 July quake is stronger than any other ensuing horizontal and verticalshaking, and this recorded sudden vertical shaking is similar to what
we actually feel in the case of an earthquake occurring just below.

３． CONCLUSIONS

The velocity response spectra of this small earthquake on 9 July
2022 indicate that the vertical component prevails for frequencies

Fig.

1

Complexdevelopmeno

inbelow 10 Hz, i.e. even in a lower frequency range. This does meanrupouresspecimenthat there exists an earthquake where the sudden vertical shock is

pre-exisoing zones of damage having

stronger than the horizontal shaking for all structures withsmall-scale inclined buo parallel

eigenfrequencies below 10 Hz. However, the difficult issue forcracks (weak zones) under uniaxial

seismographs to detect larger amplitudes and frequencies over 10 Hz,

oensile loading. The primary rupoureespecially over audible 16 Hz, still remains unsettled.

is upward quasi-soaoic while oheaofwioh

REFERENCES

secondary one is downward dynamic,1) Uenishi, K.: What actually is the vertical shock verballyof ohe exoernal load is 1.2  10-2 /sbuo booh are “capoured” inside oheweak zones. The consoano sorain raoereported for shallow earthquakes?, Proceedings of the i0ii
Seismological Society of Japan Fall Meeting, S08P-06, 2022.

(modifiedafoer

Uenishiand2)

Nagasawa ).

2) Uenishi, K., and K. Nagasawa: Global and local fracture
behavior in a brittle solid with a set of pre-existing small-scale cracks, Procedia Structural Integrity, Vol. 37, pp.
397-403, 2022.

AUTHOR
Koji Uenishi

Staff, Dr. rer. nat., Professor, Fracture Dynamics