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Simulating an Entire City
Undergoing an Earthquake
K computer achieves unprecedented accuracy insimulating quake damage,revealing a number of unanticipated results

A simulation of ground tremors and deformation of buildings in downtown Tokyo

8K computer Newsletter
March 2014 No.

Simulating an Entire City
K computer achieves unprecedented accuracy in simulating quake damage,

Quake damage prediction
has been based
solely on experience

Professor of Earthquake Research Institute,
The University of Tokyo
Leader of the Computational Disaster Mitigation and Reduction Research Unit, RIKENAICSUpon graduating from university as a civilengineering major, Hori had originallyplanned to join a major construction ﬁrm.
But he changed his mind and continued towork in the world of research, inspired by aJapanese scientist he encountered while
studying at Northwestern University.

In the aftermath of the 2011 earthquake-tsunami that devastated the Paciﬁc coastal areas of Tohoku in northeastern Japan,
criticism arose concerning the lack of eﬀective local and national government measures to cope with unexpectedly large
tremors. The Tohoku tragedy taught us an important lesson:
We need to know exactly what steps to take to prevent quake
injuries and minimize damage.
Consequently, some in academia have put a priority on taking
a scientiﬁc and rational approach to predicting the scale ofdamage possible. Hori, working with the K computer, hassought ways to integrate simulations that cover everythingfrom how the ground and buildings move and shake, to how
residents are evacuated, particularly in urban areas. In the following interview, he comments on the results of his researchand how they can help further investigations.

Fig. 1 Fragility curve

Intensity

Muneo Hori

Humans, Hori pointed out, have beenbuilding structures for thousands of years.
Yet it was not until around the time of theGreat Kanto Earthquake of 1923 in Japanthat scientiﬁc investigation into earthquake damage to structures began to be
widely investigated around the world. In
the years that followed, the concept ofhorizontal quake acceleration was appliedin the design of buildings.
Then in 1940, knowledge of seismic
waves measured during earthquakes wasemployed in structural design for the ﬁrst
time, in the United States. In subsequentyears, earthquake engineering constantly
improved and quake-resistant design
grew in importance.
Seismic wave data are now widely
accepted as an important tool for designing quake-resistant structures. A modeled
structure is subjected to earthquake simulations, and numeric data are collected on
how the model shakes and which sections are most heavily aﬀected.
But a city has so many structures that it
is impossible to model or simulate all ofDamage
The fragility curve is a statistical tool representing the relationship between the intensity of
past earthquake and the scale of quakedamage.

Undergoing an Earthquake
revealing a number of unanticipated results

Fig. 2 A simulation of ground tremors and deformation of buildings in downtown Tokyo

them despite the increasing importanceput on predicting earthquake damage. So
how are local governments estimating
possible damage?
“Theyʼre drawing on the experience of
past earthquakes and predicting damage
with the aid of a fragility curve that
depicts relationships between the intensity of earthquakes and actual damage,”says Hori (see Fig.1).
“Structures based on old architectural
quake-resistance standards can moreeasily collapse,” he notes, “which meansthat as long as past experience is theprimary tool of prediction, presumed
damage will be much greater than actually occurs in cases where modern building
standards have been employed. This is far
from being a reliable forecasting method.
Yet the simple fact is, no other means
have been available.”

A scientiﬁc, rational method
for highly eﬀective disasterprevention

So Hori and his team decided to simulate an entire city and subject it totremors̶a breakthrough method inearthquake-damage prediction. Whatenabled this previously impossible task to
be carried out is the huge computing
power of K computer. The team used thesupercomputer to reproduce the diﬀerent
ways an earthquake causes the earth tomove and structures to shake and slide(see Fig. 2).
To do this, they simulated an area̶8※An animated illustration is available in the Web edition.
Two models̶one for the geological strata or ground, the other for structures̶were created, and the K
computer calculated how each model shook. The ground model was given the same level of vibrations as in theearthquake that hit the southern parts of Hyogo Prefecture in 1995, later known as the Great Hanshin-Awaji
Earthquake. In this picture, the redder the buildings are depicted, the larger they were deformed. Deformation
exceeding a certain level is regarded as indicating the building has collapsed.

km east to west and 7.5 km north tosouth̶of downtown Tokyo, which is aheavily built-up area with more than
250,000 buildings and houses. Three-dimensional models of all these structures
were created, based on data from a
cartographer. Each of the three geological strata beneath the area̶clay, sandand bedrock̶were also modeled,using data from the Japan GeotechnicalSociety.
Horiʼs group applied vibrations asstrong as those measured during 1,000
actual earthquakes in the past to thegeological model, and computed theways the structures shook. Thisapproach proved very useful and a big
step forward in gauging the size ofdamage, that is, in identifying vulnerable
structures by the strength and type ofearthquake.
Says Hori, “Once we establish an eﬀective scientiﬁc and rational means ofdamage prediction, it will help us set
priorities in working out overall anti-disaster measures. And applying a wider
variety of diﬀerent quake vibrations to
the models will certainly contribute to
reducing the number of ʻunanticipatedʼ
situations.”

Diverse calculations
via a combination ofsimulations

Earthquake damage is not limited tostructures. The Tohoku disaster of 2011
triggered tsunami tidal waves thatclaimed a large number of lives. Such
giant waves could accompany big
quakes occurring in whatʼs known as theNankai Trough, and would wreak havoc
on aﬀected areas.
※AnWithanimated
illustration
is available
the Webthisin mind, Hori inandhis edition.
colleagues simulated a number of concurrent quake-tsunami disasters (see Fig. 3).
“We used ground and building models
similar to those created for the downtown Tokyo project, then simulated
the motion of a tidal-wave by employing a particle method,” he explains(see Note 1). “Results indicated thatbuildings whether collapsed or not
determine the direction of sea-waterinﬂow.”
The scale of a disaster is also determined by how orderly residents areevacuated. Using the K computer, Hori

Fig. 3 A simulation of a concurrent earthquake-tsunami disasterFig. 4 A simulation of peopleʼs action-patterns during evacuation(based on a multi-agent model)※An animated illustration is available in the Web edition.

※An animated illustration is available in the Web edition.

This simulation shows tidal waves swamping a coastal town. Seen on theleft is a simulation with undamaged building models, and on the rightis a case after the quake where a part of buildings have collapsed completely. One clear diﬀerence between the left and right sides is the way
waves stream in.

This simulation by K computer of 200,000 “agents” representing citizens in Kochi City
reveals some characteristic pattern of the evacuation.

simulated how some 200,000 citizens
from Kochi City on the island of Shikoku
would be evacuated to the surrounding
hills (see Fig. 4). This simulation called
for the employment of a multi-agent
model among others (see Note 2),
which Hori believed would produce
more realistic images.
“We based our calculations on theassumption that evacuating residents
would produce some identical patternof movement̶they would decide forthemselves in what directions to ﬂee
and would slow down when they wereabout to run into other people escaping,” says Hori. Results of this simulation
will help greatly in determining evacuee-guidance procedures and the installation of related road signs and guidance information.
Horiʼs group have devised separate
analysis models and computing procedures for ground-building movements,
tidal waves and resident evacuations.
Diverse data necessary for conducting
these analyses have now been collected. But Hori is looking even furtherahead. “The next major step is to integrate all these components,” he says,“so that we can conduct a greater rangeof calculations with the goal of estab-lishing simulations that produce datacloser to reality.”

A scientiﬁc approach todamage prediction urged
for all regions

Hori is also currently engaged in applying newly developed programs for structure modeling and computation to theanti-disaster projects of Kobe and othermunicipalities in Hyogo Prefecture.
“Thanks to the detailed structural data
provided by the Kobe City government,
simulations based on models moreaccurate than for downtown Tokyo will
be possible,” says Hori. “Once a scientiﬁcand rational tool is readily available,
other cities than Kobe will provide us
more detailed data, and this will
increase simulation accuracy.”
Hori is moving ahead with yet anotherambitious plan: He wants Japanese universities to use the K computer-developed programs on their supercomputers. “Predicting quake damage should
be conducted for all regions,” he argues.
“Even though itʼs impossible to performthe same level of computation in terms

RIKEN Advanced Institute for Computational Science (AICS)
7-1-26, Minatojima-minami-machi, Chuo-ku, Kobe, Hyogo, 650-0047, Japan
TEL: + 81 78 940 5555 FAX: + 81 78 304 4964 E-mail: aics-koho@riken.jp http://www.aics.riken.jp/en/of scale as K computer, university supercomputers are still competitive in termsof accuracy if the area to be modeled isreduced to, say, one-tenth. And if university machines are used strategicallyin this way, scientiﬁc and rational
damage prediction can be possible on aregional basis.”Such comprehensive simulation forpreventing and minimizing earthquake
disaster has only just begun. Hori,
always looking to push the boundaries
of supercomputing, will no doubt continue to lead in this ﬁeld for some time
to come.

Note 1The particle method, depending on
what is being calculated, simulates themotion of interacting particles such as
water-drops, molecules, etc. to track, forinstance, the complicated movement ofsurface water. Hori employed a method
normally used in computing the motionof stars.
Note 2The multi-agent model aims to simulateinteractions between people, whereeach person is viewed as an agent thatacts according to a particular rule.