Chapter 10
The Science of Earthquake Shakes
Field of expertise: Earthquake engineering, Strong motion seismology

Susumu Ohno
Translated by Catherine Sachi Kikuchi

Summary
The Great East Japan Earthquake was caused by a complex seismic source with completely
different frequency characteristics of seismic waves emitted along the trench and near land.
Constructing this type of source model for each frequency range has become an issue inevaluating strong motion of the ground. It is essential to improve the accuracy of the subsurface
model for earthquake damage prediction and evaluation of long-period ground motions, and there
is a need to construct a reliable model using a large number of observation records.
Keywords: strong ground motion, earthquake damage, long-period ground motion, subsurface
structure, source model, earthquake early warning

Introduction
Strong ground motion characteristics can be classified into three main categories: source,
propagation path, and ground amplification. In the Great East Japan Earthquake, modeling thesource of huge earthquakes, especially the subsurface structure that contributes to the perioddependence and long-period ground motions in distant areas became an issue. The method ofearly warnings was also reviewed.

1: Problems Revealed by the Great East Japan Earthquake
What happened?
The magnitude of the Great East Japan Earthquake was 9 - it was a very large earthquakethat had never been observed in Japan before. As shown in Figure 10-1, tremors of intensity 6 orhigher, and in some cases intensity 7, were observed for a long time over a wide area along thePacific coast of the Tohoku region. In general, short-period tremors with a period of 0.5 seconds orless prevailed in many places, but long-period tremors with a period of several seconds or moreprevailed in large plains such as the Kanto, Nobi, and Osaka plains due to the deep and soft
subsurface structure (Architectural Institute of Japan, 2012).

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Sendai City, Miyagi Prefecture, experienced the 1978 Miyagi Earthquake (M7.4).
Comparing the strong-motion records observed at the same location, it was found that the periodcharacteristics were very similar in periods less than 10 seconds, which is related to the shaking
damage of buildings and ground. The amplitude was about 30% higher in the Great East Japan
Earthquake, which is smaller than the difference in seismic magnitude scale. However, theduration of the earthquake ground motion was much longer (Architectural Institute of Japan, 2012).
!"#$#%&'"()%*#$#%&+,$-%&./.0$1,1'#23$-"/4"$#1'/2%'#1$'"#$#5/4#.'#&$+64%'/6.$%.7$#%&'"()%*#$14%+#$8%1#7$6.$'"#$681#&9#7$/./'/%+$'&#26&1$%.7$/11)#1$%$-%&./.0$8#:6&#$'"#$2%;6&$'&#26&$%&&/9#13$).7#&#1'/2%'#7$'"#$#%&'"()%*#$14%+#$8#4%)1#$'"#$14%+#$6:$'"#$/./'/%+$&)5')&#$'"%'$
5&#4#7#7$'"#$2%;6&$1+/5$-%1$12%++<$Figure 10-1. Seismic intensity distribution of the Great East Japan Earthquake
The reality of the damage2

As for the damage caused by the tremor, in Miyagi Prefecture and other areas close to theepicenter, buildings constructed according to old standards collapsed or were badly damaged andhad to be rebuilt, and many old residential areas collapsed. In Sendai City, which also experienced
the 1978 Miyagi Earthquake as mentioned above, the areas that experienced major damage in2011 generally overlapped with that of 1978, suggesting that the effects of ground response weresignificant. Ground subsidence damage due to liquefaction occurred in a wide area, especially in
the Kanto region.
Long-period ground motions prevailed in the large plains, resulting in super high-risebuildings being exposed to tremors for a very long time. At the Sakishima Government Building on
the coast of Osaka Bay, 700 kilometers from the epicenter, the natural period of the building
resonated with the long-period seismic motion, resulting in shaking of over 1 meter at the top of thebuilding. Even in cases where there was little structural damage that threatened the safety of thebuilding, there were cases where secondary materials called non-structural materials, such asceilings, partition walls, and equipment sustained significant damage, pointing out the importanceof ensuring not only structural safety but also interior safety.

2: Paradigms Destroyed by the Earthquake
Conventional wisdom and necessary responses
The assumed epicentral region and earthquake magnitude have been used in seismic
hazard assessments for building design and earthquake damage estimation. Since the 1995 Great
Hanshin-Awaji Earthquake, it has not been enough to simply set up a uniform slip on the fault
plane to evaluate the ground motions in the period range, which are important for structures. It has
been important to model the strong ground motion generation area, which is a place that emits
especially strong ground motions, and this kind of source has been modeled for major
earthquakes.
In the Great East Japan Earthquake, multiple source regions, which were thought tocollapse independently, were linked together, resulting in an earthquake that was much larger thanexpected. At the same time, there was a large amount of slippage in shallow areas along thetrench in the very long period (period: 20s or more), which caused large tsunamis and crustaldeformation. On the other hand, in the period that caused seismic damage (period: less than 10s),
slippage in deeper areas closer to the land was larger. The existence of such a difference in theperiod range has been pointed out before, but it had not been incorporated into the scheme ofstrong motion evaluation because of few observations.
As for long-period ground motions, it has been recognized that they can cause damage
even in distant areas due to resonance with structures, as typified by the damage to oil tanks in
Tomakomai, Hokkaido, after the 2003 Tokachi earthquake, but it had not been assumed that asuper-massive earthquake such as the Great East Japan Earthquake would cause significant
shaking of the super high-rise building on the coast of Osaka Bay, 700 kilometers from theepicenter.
Since the earthquake early warning system estimates the epicenter as a point source, it
had been noted that there was a problem in its applicability to huge earthquakes where faults
rupture over a wide area, but this problem had not yet been solved.

3: A New Approach3In order to incorporate the main slip areas, as described in section 2 above, differ
depending on the period into the strong ground motion evaluation scheme, it is necessary tocontinue studying the modeling for each period. It has also been pointed out that the same strongmotion generating region is not sufficient even for the period that affects the ground motion, andthat more hierarchical modeling is necessary. The Great East Japan Earthquake was a huge
subduction-zone earthquake, but even in inland earthquakes such as the 2016 Kumamoto
Earthquake, there is a difference between a case where the fault does not reach the groundsurface (buried fault) and where it appears on the ground surface. In the former case, fast slip in
deep areas generates short-period components, while in the latter case, slow slip in shallow areas
dominates the long-period components, and the direction of seismic motion dominance is different
from the former. The detailed source modeling of the different period ranges and the incorporationof such modeling into the evaluation scheme of ground motions for design are still insufficient due
to the small number of observed cases, but the impact on long-period structures is particularly
significant, and further studies are necessary.
The accuracy and spatial resolution of the subsurface structure model needs to beimproved because the subsurface structure has a significant influence on the evaluation of longperiod ground motions in addition to the modeling of the source. However, after the Great East
Japan Earthquake, the National Research Institute for Earth Science and Disaster Resilienceestablished a seismic, strong-motion, and tsunami observation network (S-net) for areas in theocean, and it is expected to improve the accuracy of the subsurface model for evaluating strong
ground motions using this network. We must improve the accuracy of the subsurface model for theland areas as well, because the area where the earthquake damage is concentrated often overlaps
with the area that the amplification is large in the period (period: more than 1 second for low-risebuildings, longer period for seismically isolated buildings and super high-rise buildings), which has
a large impact on structures.
Compared to buildings, the strong-motion observation of the ground has been enhanced by
the nationwide seismic intensity observation network and strong-motion observation network sincethe 1995 Great Hanshin-Awaji Earthquake, but the spatial resolution of the shaking obtained at thetime of disaster is still low. In order to improve the spatial resolution of the shaking at the time ofthe disaster, it is necessary to increase the density of the strong-motion observation itself. And inorder to improve the accuracy of spatial interpolation, it is necessary to improve the accuracy andspatial resolution of the underground structure model.
In the case of early warnings for earthquakes, the estimation of the epicenter as a point
source was a cause of underestimation, so a method is being developed and applied to estimate
tremor wavefields, rather than a method to estimate the epicenter from the initial tremors (JapanMeteorological Agency, n.d.).

4: Achievements and the Future
A new approach to disaster science
In order to obtain accurate ground motion for design purposes and earthquake shaking
distributions, we must improve the accuracy and resolution of subsurface models. To do this,subsurface structure modeling has been widely carried out using geophysical exploration andmicrotremor measurements. We are trying to improve a modeling method of subsurface structuresusing strong motion observation records based on a joint method, which has advantages such as
direct optimization of 3D structure and simultaneous optimization of multiple records.

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In addition, since seismic activity in Japan is one of the most active in the world,
observation data of strong motion are accumulated every day, and a data-driven approach ispossible. In order to improve the accuracy of strong-motion evaluation and shorten the prediction
time, we are using machine learning to conduct research on predicting strong-motion for design
and immediate shaking (Figure 10-2) (Ohno & Tsuruta, 2018).

Figure 10-2. An example of earthquake motion evaluation by deep learning (Ohno & Tsuruta,2018)

Conclusion - from the author
In recent earthquake disasters, such as the Great East Japan Earthquake, areas wheredamage is concentrated sometimes appear. In most cases, such damage concentration occurs
where there are many buildings constructed based on old standards in the area where the groundamplification is high in the period where the structures are likely to shake. It is necessary tounderstand in advance where buildings are likely to shake based on observation data and reflect itin the ground motion design. It is often pointed out that the evaluation accuracy of strong groundmotions is lower than that of a building response, but many records of strong ground motions haveaccumulated so far, and it is necessary to improve the evaluation accuracy by using data-driven
approaches.

References
Architectural Institute of Japan. (Ed.). (2012). Preliminary Reconnaissance Report of the 2011
Tohoku-Chiho Taiheiyo-Oki Earthquake. Springer. https://doi.org/10.1007/978-4-431-54097-7
Japan Meteorological Agency. (n.d.). About the PLUM Method
https://www.data.jma.go.jp/svd/eew/data/nc/plum/index.html

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Ohno, S., & Tsuruta, R. (2018). Ground-motion prediction by ANN using machine learning for theTohoku region, Japan. In 11th National Conference on Earthquake Engineering 2018, NCEE 2018:
Integrating Science, Engineering, and Policy (pp. 5429-5437). (11th National Conference on
Earthquake Engineering 2018, NCEE 2018: Integrating Science, Engineering, and Policy; Vol. 9).
Earthquake Engineering Research Institute.

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