(gentle music)
Kumamoto Castle
is known as one of Japan's three great castles.
It stood for 400 years,
capturing the people's hearts with its majestic beauty.
(gentle music)
(rumbling)
However, it was rocked by two violent tremors
during the magnitude 7.0 Kumamoto earth quake.
(gentle music)
The stone wall supporting the castle
collapsed in numerous places.
Kumamoto Castle suffered the worst level of damage
in its cultural properties
for the first time in years after World War II.
How shall the castle be restored?
(propeller whirring)
Using a drone equipped with a high resolution 4k camera,
NHK sent out to survey the damage.
The image then underwent over 3,000 hours of analysis
using the latest technology.
The result was used to reproduced
a virtual castle.
That led to unexpected discovery.
While many of the section of the stone walls
were built after the Meiji Era sustained damaged,
90% of the original stone walls withstood the earthquake.
Why were the older stone walls
more resistant to tremors?
What became apart from their construction method
was a mathematical formula
that even astounded today's scholars.
When construction originally began in Kumamoto Castle,
towards the end of the 16th century,
Japan was experiencing frequent major earthquakes.
The samurai ancestors
devised many technological innovations
to prepare for natural disasters.
Kumamoto Castle is finally revealing
its 400-year-old secrets.
How was the sturdy castle constructed?
And how should a castle full of samurai wisdom
be brought back to life?
The challenge to restore Kumamoto Castle begins.
(dramatic music)
The Kumamoto earthquake was a series of earthquakes,
including two jolts that recorded a maximum seven
on Japan's seismic intensity scale.
It took the lives of over 200 people
and forced more than 100,000 people to evacuate.
The castle complex is situated
on an extensive 242-acre campus.
How much damage did the earthquakes cost?
NHK, the Japanese public broadcaster,
cooperated with Kumamoto City
who is the administrator of the castle
in surveying the damage.
The stone walls that support the structure
had collapsed in many places.
Because of the dangerous conditions,
it was impossible to complete the survey of the damage
from the ground.
(propeller whirring)
So a drown equipped with a high-resolution 4k camera
was used to capture images of the damage.
This allowed the team to survey even the smallest damage
in places where it was too dangerous to approach.
(upbeat music)
Based on the GPS data,
the collective image were digitized.
The analysis of the entire dataset
took more than 3,000 hours.
The result was used to reproduce
a high-precision virtual castle in 3D.
The model could be studied from various angles
to reveal even minor details.
The extent of the damage became apparent for the first time.
The main tower made of two keeps
is the castle's central structure.
The tiles had fallen off
and weeds are growing on the roof.
The stone wall had collapsed,
blocking the entrance.
(gentle music)
The Higashi Jyuhachi-ken Turret,
listed as one of Japan's most important cultural property
was destroyed.
The building had collapsed
along with the stone wall.
It was a large scale collapse,
displaying 15,000 cubic meters in volume,
an equivalent of 7,000 metal barrels.
All 13 important cultural assets within the castle complex
were damaged.
The stone walls collapsed in more than 50 places,
dislodging more than 20,000 pieces of stone.
The five-story Lidamaru Turret
is situated south of the main tower.
Most of the stone wall under the turret had collapsed.
The 10-ton structure
was barely being supported by one narrow stack of stones.
The building floor was sinking form its own weight.
When measured by a computer,
the warp in the floor was 55 centimeters.
The entire building was in imminent danger of collapse.
Yet, matters were getting even worse.
There were many aftershocks.
They amounted to more than 3,500 in a month.
The situation remained critical.
An emergency operation
to limit further damage to the building began.
Would this intervention
prevent the structure from collapsing from the aftershocks?
A giant steel support arm
was carefully moved toward the turret.
The tip of the arm will be slid carefully under the building
so that the scaffold would cradle
and support the weight of the turret.
The work continued, adjusting the position of the arm,
several milimeters at a time.
They're now ready to insert the giant arm
where the stone wall had collapsed.
(gentle music)
One week after the operation had begun,
the arm that supports the weight of the turret was set.
That allowed access beneath the turret.
Though the aftershocks continued,
the spread of damage had been halted for the time being.
Toshihiro Tsumagari works for the municipality
of Kumamoto City.
He is the project leader for the reconstruction.
Kumamoto City proclaimed their goal
to rebuild the main tower,
the castle's symbol, in less than three years.
It was because the main tower holds a special place
in the hearts of the Kumamoto citizens.
There's a reason why the main tower
hold such a special place in the hearts
of the Kumamoto citizens.
(gentle music)
Kiyomasa Kato was the first lord of Kumamoto Castle.
He built the main tower 400 years ago.
(explosion booms) (guns firing)
But the Seinan War or the Satsuma Rebellion
broke out during the Meiji Era.
The main tower was burned to the ground.
For more than half a century,
Kumamoto Castle was without a main tower.
In the Showa Era,
Kumamoto came under attack during World War II.
(explosions booming)
Many people lost their lives in air raids.
(gentle music)
Then in 1959,
plans were announced to restore the main tower
as a symbol of post-war reconstruction efforts.
Donations from citizens poured in,
and the reconstruction began.
The blue prints from that reconstruction had been preserved.
In the rebuilding efforts,
many Kumamoto citizens participated.
This is Yoshio Miyazaki.
(speaking in Japanese)
His late mother worked on the castle.
His mother, Suwaku, worked at the local lumber mill.
She volunteered when she heard
the main tower was being rebuilt.
She was put in charge of transporting roof tiles
and other construction materials.
Kumamoto Castle was brought back to life
in an effort to heal the scars of war.
The castle stands as a symbol
of the dedication and perseverance
of the citizens of Kumamoto themselves.
The city of Kumamoto announced their goal
to restore the main tower
before the third anniversary of the earthquake.
Six months after the quake,
the mayor inspected the main tower.
There was no major damage to the parts of the structure
built with reinforced concrete.
But the stone wall supporting the tower had collapsed,
and the stones had slid into the building.
Here is the entranceway
where many visitors used to come and go every day.
A technical proposal created by a construction company
to rebuild the main tower was submitted to Kumamoto City.
It was a proposal to rebuild the castle
with the elaborate earthquake resistant reinforcements.
First, a portion of the stone wall
would be dismantled and removed.
Next, the pilings and other structural supports
would be made more earthquake resistant.
After that, the stone walls would be rebuilt.
However,
Tetsuo Tanaka of the
Himeji Center for Research into Castles in Fortifications
complained that deliberately damaging
architectural artifacts, even temporarily,
was out of the question.
Another colleague, Hiroshi Kitano,
of Tohoku University of Arts and Design,
was also very critical of the proposed plan.
How could the mayor preserve the work
of cultural properties,
yet make the structures quake resistant?
Kumamoto City had to reconcile these two opposing interest
before they could start reconstruction on the castle.
In the mean time, NHK continued its investigation
into the damage to Kumamoto Castle.
Analysis of the three dimensional model
revealed something quite unexpected.
Uto Turret
was built during the castle's original construction.
(gentle music)
Although it was one of the oldest castle structures,
it withstood the earthquake.
Amazingly, the wooden structure
that had stood for 400 years
suffered no major damages.
The pillars and beams remained as they were
when they were first built.
The secret lay in the stone walls
that form the foundation of the structure.
When compared to the pre-quake survey data,
there was almost no deformation.
The stability of the stone wall
minimized the damage to the turret.
The neo stone wall is also one of the oldest structures
that have survived to this day.
It, too, has minimal deformation.
Was it just a coincidence that the original stone walls
withstood the force of the earthquake?
To solve this mystery, we looked into the history
of the stone walls.
During the 400 years since its construction,
Kumamoto Castle has gone through
numerous additions and restorations.
When the locations of the collapsed
from the present earthquake are mapped,
we see that 31% of the damage
occurred on stone walls restored in the post-Meiji Era.
Meanwhile, damaged to the walls
of the castle's initial construction
was limited to 10%.
90% of the samurai stone walls withstood the quake.
Kumamoto castle was built by Kiyomasa Kato.
He is known as a master castle builder
who was involved in the construction of many castles.
Why were the stone walls from the original construction,
the ones that Kiyomasa built, so sturdy?
To learn the secret,
we've asked experts to examine the 3D model.
Tomio Tamano is an expert on stone wall structures.
He believes the secret of why the stone walls
are resistant to earthquakes
lies in the way the stones were stacked.
Tamano focused on the angle of the stone.
Prior to the construction of Kumamoto Castle,
stones were stacked parallel to the ground
in most Japanese castles.
But in Kumamoto Castle,
stones are stacked at a 90 degree angle to the slope.
Stones stacked parallel to the ground
have a tendency to break loose
when sideways forces are exerted by an earthquake.
On the other hand,
When the stones are stacked at right angles to the slope,
the force is dispersed,
making it harder for them to dislodge.
(speaking in Japanese)
Another expert focused on the entire construction
of the stone wall.
Soichiro Kitagaki has been studying Japanese castles
for over 50 years.
Nori-gaeshi is the name given to the steep curve incline.
The intact neo stone wall is one example of this.
It's called musha-gaeshi
or samurai proof.
Musha-gaeshi is said to have been invented
by Kiyomasa Kato.
It is so named because it looks as tough it can repel
invading enemies
who are about to attack the castle.
It's the symbol of the impregnable castle.
Using simulation, we test a linear gradient
without musha-gaeshi.
The deduced forced applied to the stones by the tremor
is indicated by the arrows.
We see there's a large outward thrusting force
on the stones.
Now, the musha-gaeshi wall.
The force works along the surface of the stone wall.
The result clearly shows
that it's harder for the stone to break loose,
making it more resistant to earthquakes.
But while the original stone walls
mostly withstood the force of the earthquake today,
why was there so much damage to the stone walls
that were restored in the post-Meiji Era.
The researchers believed that the steep linear gradient
is one of the main factors.
Here is one example.
Hohoate Gomon Gate
was restored in the Meiji Era.
It suffered catastrophic damage from the earthquake.
The gradient of the wall they restored
was straight and almost perpendicular,
which we can see in the photo
shot before the 2016 Kumamoto earthquake.
The experts don't know
why they didn't use the musha-gaeshi curve.
The difference in the way the stones were stacked
changed the fate of the building hit by the earthquake.
During the course of the research,
the crew learned of the existence of an old book
with building instruction from musha-gaeshi.
The Secret Book of Stone Walls,
Ishigaki Hiden No Sho in Japanese.
It is said that the book was written
by Kiyomasa's stone wall building artisans.
First, to calculate the nori curve.
When calculating a 50% nori, multiply six by five
to get a three shaku nori.
Nori,
this unfamiliar word is repeated often in the document.
What does it mean.
Kitagaki was able to decipher the secret document.
First, he divided the stone wall into one ken,
or 1.8 meter sections.
Then he drew a small triangle.
Nori refers to the slope of this triangle.
The nori slant is determined by the length
of the base of the triangle.
By shortening the base
using the same formula as you go up the stone wall,
the nori wall curves back on its self.
It requires a very complex modern mathematical formula
to come up with this gradient.
It is now clear that the stone walls
were constructed to exact specifications.
400 years ago, samurai warriors
were living in a tumultuous age
of Japan's civil war.
They lived in constant danger of enemy attacks.
(weapons clanking)
Was musha-gaeshi invented to repel the enemies?
Could it be the Kiyomasa invented the structure
as an anti-quake measure?
Coming up, we'll unfold these mysteries.
(speaks in Japanese)
We'll also revisit the reconstruction efforts
at the castle today.
How will they make the castle more quake resistant
while preserving its value as a cultural property?
We'll be on-hand to witness the unprecedented challenge.
One expert has taken up the challenge
to solve the mystery surrounding the invention
of musha-gaeshi.
He is Soichiro Kitagaki.
Kitagaki headed for another castle created by Kiyomasa
to investigate.
The ruins of Ulsan Castle are situated on a mountain top
near Ulsan City in the southern part of South Korea.
(melodious piano music)
The curved incline was not utilized in Ulsan Castle
which was built by Kato Kiyomasa in 1593.
He built Kumamoto Castle six years after Ulsan castle.
So, the earthquake resistant curve
must have been invented in the interim.
What happened in the six years
between the two castle constructions
was the Keicho-Fushimi earthquake.
A huge earthquake swayed and damaged
every part of Fushimi Castle.
Fushimi castle suffered catastrophic damage
from the earthquake.
In this castle, Kiyomasa experienced a massive quake
that is said to have killed more than 1,000 kiyodowite.
In the aftermath, it is thought
he devised the anti-quake measure.
(speaking in Japanese)
Experts confirmed that at least 17 sections
of the Kumamoto Castle stone walls
matched the complex mathematical formula
deciphered from the Secret Book of Stone Walls.
Besides the musha-gaeshi curve,
there was another new method employed
in the stone walls of Kumamoto Castle
to make them even more quake resistant.
It was the well engineered angle
in which the stones were stacked.
Prior to building Kumamoto,
stones were stacked parallel to the ground
when building most castles.
But in Kumamoto Castle,
stones are stacked in a 90-degree angle to the slope.
This is what made the walls more quake resistant.
The experts investigated the changes
in the stacking angle.
This is the data of various castles around Japan.
Before the Keicho-Fushimi earthquake,
the stone walls were arranged parallel to the ground.
But after the Keicho-Fushimi earthquake,
the stones in Kumamoto castle were the first to be stacked
almost perpendicular to the slope,
raising the level of quake resistance remarkably,
and there after.
This technology was adapted in castles all around Japan.
(gentle music)
Kumamoto castle marked the evolution
of the castle construction technology in Japan.
To build the Kumamoto stone walls,
Kiyomasa called upon the artisans
who had experienced the Keicho-Fushimi earthquake.
By trial and error,
they attempted to build a sturdier stone wall.
(speaking in Japanese)
Work towards restoration has begun at the castle.
The collapsed stones were being carried away
by stone workers.
The work progresses with utmost care,
so as not to damage the stones.
These stones will be used to rebuild the stone walls.
They're all numbered and will be carefully stored.
(gentle music)
The stones amounted to more than 20,000.
Since no blue prints of the stone walls exist,
how will the team rebuild the walls?
NHK decided to team up with a researcher
to create a new software to facilitate restoration.
It's programmed to instantly show
the most likely place where a stone may have been positioned
in a stone wall.
(suspenseful music)
When you trace the outline of a stone's flat surface,
the program recognizes and computes the shape.
The screen shows where the stone
is most likely to have been located
before the earthquake.
This method of determining the location
of each of the 20,000 stones is continuing.
As the reconstruction of Kumamoto Castle began,
a new risk factor came to light.
Even in the stone walls that escaped damage,
bulging parts were found.
The entire center section of this stone wall
was also bulging.
Three weeks after this bulge was discovered,
it suddenly collapsed.
The damage is spreading unseen
through the internal structure.
Tatsauki Nishigata who study stone walls structures
has been investigating the bulges.
This is Nishigata's analysis
of why the stone walls have bulged.
The inside of the stone wall
is stuffed with cobblestones.
Previously, they were thought to absorb
the sway of the quake
and stabilized the stone wall.
But the two major Kumamoto tremors
have been followed by countless aftershocks
for an extended period of time,
increasing in recent years.
These frequent tremors
have caused the cobble stones to sink,
forcing the wall stones outward.
When the outward force becomes too great,
the wall collapses.
(rumbling)
As the investigation progressed,
the original stone walls
thought to be more resistant to earthquakes
were also to discovered to have some bulges.
In the entire castle complex,
no less than nine sections have been found to have bulges,
and two of them were in imminent danger of collapse.
Analysis was conducted on one of the two sections.
The concentration of red
indicates a high probability of collapse.
If even a portion of the wall collapses,
it could cause a chain reaction
that may lead to large scale failure.
What if the damage gets even worse?
For the Kumamoto citizens who have devoted their energies
to speedy reconstruction,
this is extremely alarming.
If the proposed reinforcement works are applied
by using cutting-edge technology,
the stone walls will be more quake resistant,
but that can take away from its worth
as an important culture property.
The mayor was once again faced
with a difficult dilemma,
how to preserve the historical value of the castle,
yet strengthen it to withstand earthquakes.
Kumamoto Castle experienced two great quakes
after Kiyomasa's death.
They took place in 1619 and 1625.
A stone wall that was restored during those earthquakes
has survived to this day.
The restored wall is known for its musha-gaeshi curves.
The right curve is attributed to Kiyomasa
and the left is the extension Tadatoshi built.
Builders at the time placed large stones
at the curved corner of the wall.
This was to make Kiyomasa's musha-gaeshi wall even sturdier.
By using larger stones,
the contact surface areas increased,
making the structure more stable.
Mayor Onishi has been trying to cope
with the conflicting demands
in preserving the value of the cultural property
and the need to make the structures more quake resistant.
As the further risks of earthquake damage loomed,
the cultural experts also started to feel
that there is a need to make the wall
more resistant to earthquakes.
Specifically, concerning stone wall reconstruction,
the experts proposed incorporating
some of the old technology chronicled
in the Secret Book of Stone Walls.
As to the bulges in the stone walls,
they debated the use of modern measures,
such as special sheets
to prevent the cobble stones from sinking.
Finally, the construction to rebuild the castle
began in a full scale.
(birds chirping)
A year after the Kumamoto earthquake,
the castle grounds that have been off limits
was partially open to the public.
(gentle music)
With the wisdom of their samurai predecessors,
the major and experts perceived
with the most important issue in mind,
the protection and preservation of human life.
Restoration of Kumamoto Castle
will continue for years to come.
(gentle music)