August 19, 2013 | 3
“Imagine standing on the roof of a three storey building and lowering the lead of a mechanical pencil so that it hits the center of a coin sitting on the ground. That is what we had to do.”
At 14:46 on March 11, 2011, a magnitude 9 earthquake ripped through the earth off the Pacific Coast of Tohoku, 231 miles northeast of Tokyo. The most powerful quake ever to have hit Japan, the Tohoku earthquake triggered a 40.5 meter tsunami wave, moved the main island of Japan eight feet east and left tens of thousands dead. As a nation tried to rebuild their lives, the word on everyone’s lips was ‘why?’
It was this question that drove Sanny Saito from the Japan Agency for Marine-Earth Science and Technology to join the research expedition that would combine scientific expertise from ten different countries onboard Japan’s scientific drilling vessel, Chikyu. Meaning ‘Earth’ in Japanese, Chikyu is the largest research ship in the world, weighing in at 57,000 tons and measuring 210 meters in length.
“Chikyu is like Japan’s spaceship!” Saito describes.
However, Chikyu’s mission is not to explore extraterrestrial worlds. With the ability to drill a staggering 7 km below the seafloor, Chikyu’s purpose is to uncover the hidden world of our own planet beneath ocean’s surface. The ship is one of the main platform for the Integrated Ocean Drilling Program; an international marine research initiative dedicated to understanding the Earth by exploring the environment below the seabed. One year after the Tohoku earthquake, Chikyu set sail to retrieve a sample of rock from the exact place the quake occurred. By being able to see the geological structure at the quake site, scientists hoped to uncover vital information to help us understand major seismic events.
This investigation was particularly important because the scale of the Tohoku earthquake was surprising. It was not just because the event was the 5th most powerful quake since records began in 1900, but because no one predicted that it should have been that large.
Situated on what is known as the ‘ring of fire’, Japan is no stranger to earthly shakes. At the point where the Tohoku earthquake originated, the rocky plate on which the Pacific ocean sits is attempting to burrow under the continental plate of Japan. Normally, this motion creeps forward at 8.5 cm each year, but occasionally the plates will suddenly slip due to the pressure build-up behind locked zones of rock that are stubbornly resisting this movement. The slip is felt as an earthquake and its magnitude is governed by the size of area that moves and how far it travels. Seismologists studying the region believed that these slips should be relatively small, but the reality was a 50 metre displacement that slammed into the trench of the northern Pacific ocean, triggering a tsunami. Such a large slip into the Pacific trench had never before been seen and scientists were at a loss to explain why it had occurred.
The problem with trenches is their definition demands that they are deep. Before Chikyu could start investigating what lay below the seafloor, it had to extend its drill pipes almost 7 kilometers down through the ocean; a depth that would make this the deepest sea operation ever to have drilled so deep.
As the pipes were extended into the water, Chikyu used thrusters controlled by satellite GPS signals and acoustic waves from the sea floor to hold its position during the drilling exploration. Since the drilling would take almost two months to complete, it was vital that the ship be able to remain stationary even in high winds or currents.
In the first stage of the process, a wide steel pipe was planted into the seafloor to act as the drilling well entrance. Pressure alone is sufficient to push the well entrance into the top layer of the seabed, after which the pipes between the ship and seafloor were retracted to allow the drill head to be connected to their base. This is when it became more challenging; the well entrance was 50 centimeters in diameter, 7 kilometers below where Chikyu was floating. Getting the drill head into that hole was the proposition Saito compares to the mechanical pencil and the coin.
“It was a really really difficult situation,” he explains. “In Japanese, we call this a ‘kamiwaza’ or ‘act of God’.”
Errors meant the team had to perform this incredible technological feat not once, but three times. The first time the well head was placed into the seabed, the lowering pipes failed to disconnect and the operation had to begin again from scratch. The second time, the drill head entered the new well entrance only for a pipe to break. In the third attempt, the optical fibre communication system died, leaving Chikyu’s team blind. This was not to be an expedition for the weak hearted.
Once the drilling could begin, the second stage of the process commenced; that of locating the site where the plates slipped to cause the earthquake. As the pipes plunged into the seafloor, sensors mounted along their length sent a stream of information back to the scientists onboard Chikyu. One of these readings was the level of natural radiation found in rocks. If the rocks are broken up as might be expected at the location of the slip, the level of detectable radiation rises. This should be accompanied by a decrease in the rock’s electrical resistivity, since the fractured shale contains a higher water content which is a far better conductor than solid material. The final clue is the measure of the force needed to turn the drill. A constant torque means the rock is easily moving aside, suggesting the drill is passing through a low friction, easy to slip, region.
At 820 meters below the seafloor, the data streaming live back to ship marked a clear area that neatly ticked the above criteria. Chikyu had reached the earthquake slip zone.
While these sensors were informative about the slip conditions, what scientists really needed was to see the rock for themselves. Even after locating the slip point, the extraction of rock back to the ship is a precarious operation. The center of the drill head is hollowed out to allow a core of material to be taken inside the pipes and pulled to the surface. Yet due to the fragile nature of the rock, a 10 meter advance may only yield a one or two meter core, making it likely that the key sample could be entirely missed. As the drill drew near the slip, Chiyku engineers began to reduce the advance to two metres steps.
“We didn’t want to come home without the answers,” Saito elaborates as he recalls the anxiety the Chikyu team felt as they examined the cores drawn up to the ship. “People were relying on us to understand how this happened.”
And time was running out. On day 51 of the 54 day expedition, the team witnessed a spectacular event of a very different sort; an annular eclipse which sees the Moon and Sun perfectly aligned, but with the Moon’s apparent size smaller than that of the Sun, leaving a prophetic fiery ring in the sky.
“It was like a golden drill pipe,” Saito jokes. “We thought perhaps we would still be able to do this.”
Within 6 hours he was proved to be right. As the17th core was pulled onboard, the team finally laid eyes on the broken, fractured surface of the plate boundary.
“We called this the miracle core,” Saito says in describing the team’s reaction.
Under analysis in the laboratory, the answers finally started to appear. The material at the slip point consisted of 80% smectite; a clay mineral which holds moisture and can act as a lubricant. This was an astonishing find and explained the low friction measurements. The Tohoku earthquake had been so catastrophic because the mineral composition where the plates met had allowed them to skid over one another in an overshoot that hit the Pacific trench.
While this provided a large part of the answers they were searching for, the Chikyu team were not yet finished. Just over one month later, Chikyu returned to the quake site for an even more ambitious measurement; that of the temperature at the slip point.
During the earthquake, friction between the sliding plates sends the temperature soaring to 300 or 400 degrees. Such a massive elevation is detectable even a year after the event and provides new evidence for the conditions at the moment the huge slip occurred. Buried deep in the ocean floor with many chances for the heat to leak away, this measurement was Chikyu’s most delicate and the observatory did not finish its analysis until it was collected last April. Scientists are currently pouring over the data which is the first to have ever been recorded only one year after a great earthquake.
For Chikyu, the mission is complete but its challenges continue as it heads for the first exploration of the Earth’s mantle. For the scientists involved in the core extraction, their work into the origin of great earthquakes continues, but this time with the addition of one of the most ambitious samples ever collected.
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