Professor Motoyuki Kido of IRIDeS and his team are the first in the world to elucidate the complex movements of the seafloor after the Tohoku-oki great earthquake.

Disaster Science Division : Professor Motoyuki Kido

On March 11, 2011, along the Japan Trench—deep in the off-Miyagi Pacific Ocean floor—the plates (bedrock covering the earth’s surface) began to move each other, causing a huge earthquake with a magnitude of 9.0. This earthquake, and the resulting tsunami, caused severe destruction, particularly in the Tohoku region. Experts have named the “2011 Tohoku-oki earthquake” as a genuine natural phenomenon. “The Great East Japan Earthquake” is the name of the disaster that caused damage to people and society.

Due to major movements within the Earth, oceanic plates constantly sink into landward plates around Japan. Moreover, due to the friction between the plates while sinking in, strain is built up within the plates; therefore, from time to time, earthquakes occur to release this strain by rupturing the fault. Although the 2011 Tohoku-oki Eq. was also based on this principle, the rupture area of the fault was extremely large ever observed in Japan’s history.

The research team of IRIDeS, including Prof. Motoyuki Kido, and Prof. Ryota Hino of the Graduate School of Science, Tohoku University (IRIDeS concurrent faculty) used a special instrument to observe the huge movements at the time of the earthquake. In addition, over the course of four years from 2012 after the earthquake, they expanded the geodetic network and continued to investigate the seafloor crustal movement across the entire area of offshore Tohoku. That is to say, the team explored aftermath of the 2011 Tohoku earthquake. Prof. Kido and his team were the first in the world to elucidate the complex movement of the seafloor after the great earthquake. This was published as a paper in 2017^*1 , causing a great stir both domestically and internationally.

*1: Tomita, F., M. Kido, Y. Ohta, T. Iinuma, and R. Hino, “Along-trench variation in seafloor displacements after the 2011 Tohoku earthquake,” Science Advances, 3, e1700113, doi:10.1126/sciadv.1700113, 2017.

Seafloor survey for which advanced technology is required

How is it possible to investigate deep sea floor movements? For land positioning survey, measurements over ground were performed in the past; however, in recent years, with the development of global positioning systems (GPS) using artificial satellites, this investigation can be conducted in an accurate, simple, and economic way using radio waves. Changes over time, in a particular point location, can be continuously monitored, making it possible to accurately ascertain the extent of crustal movements over a specific period. In contrast, as radio waves do not reach points under the sea, a GPS cannot be used on sea floor. The area around the Japan Trench is far from the land and located at a depth of 5000 m or more from the surface of the water. It has been extremely difficult to gauge, over many years, the detailed movements of a deep seabed that is inaccessible to humans.

However, to elucidate the mechanism of an earthquake occurring beneath the seabed, it was essential not only to make observations from land that is far away but also to collect data from positions close to the seabed. Prof. Kido and the team had already realized the importance of this and were tackling this issue since the early years of the 21^st Century. In cooperation with the Japan Coast Guard, they developed and deployed a seafloor geodetic observation system, particularly focusing on the off-Miyagi earthquakes that occurred periodically. Therefore, the system was in use for observing geodetic fluctuations already in 2010, just before the Tohoku-oki Eq. occurred.

[Figure 1]

Operation to install seafloor precision transponders deploying into the sea

The equipment used by Prof. Kido and his team to measure the movements of the seabed is called a “seafloor precision transponder” [Figure 1]. With one set containing three or more seafloor precision transponders, these sets were deployed in the seabed in advance, and sound waves were sent from a sea vessel whose position was monitored by GPS [Figure 2]. The time taken for the response to come back from each transponder was measured, and by correcting for influences on the speed of the sound in ocean, due to seawater temperature change, the position of the transponder was obtained. This measurement was conducted regularly to investigate the extent to which the transponder deployed into the seabed had moved since the previous investigation and to clarify the distance and direction of the crustal movement. Fundamentally, this was the principle involved for the continuous and accurate investigation of the positional changes of the transponder, and although it seems simple, it involved water depths of 5000 m. At this depth, extremely sophisticated equipment is required to accurately grasp movements that occur in the order of centimeters.

[Figure 2] Sending sound waves to the transponder from a vessel (vessel position measured by GPS)

The 2011 Tohoku-oki Eq was completely unexpected, in terms of its scale and the area covered, compared to the expected off-Miyagi earthquakes. Nonetheless, as the seafloor geodetic observation system was already in operation, the world was able to grasp for the first time that—as a result of this earthquake—the landward plates had shifted east by up to 31 meters^*2. Until this point, the maximum shift observed on land had been a mere 5 meters near the Oshika Peninsula, positioned on the ocean side [Figure 3]. If this observation system had not been in place, we would not have been able to capture this major shift in the seabed.

*2: Kido, M., Y. Osada, H. Fujimoto, R. Hino, and Y. Ito, “Trench-normal variation in observed seafloor displacements associated with the 2011 Tohoku-Oki earthquake,” Geophys. Res. Lett., 38, L24303, doi:10.1029/2011GL050057, 2011.

[Figure 3] Movement of the seabed due to the 2011 Tohoku-oki earthquake (Kido et al., 2011, GRL)

The significance of the research of Professor Kido and the team

[Figure 4] Complex movements of the seabed over four years after the 2011 Tohoku-oki earthquake（Tomita et al., 2017, Science Adv.）

The achievement of Prof. Kido’s research team on this occasion was elucidating the “four-year seafloor movements following the 2011 Tohoku-oki Eq.” According to their research, at the time the earthquake occurred, a huge fault slip occurred off the coast of Miyagi Prefecture, which resulted in the plates continued to move to the west. In contrast, in the Fukushima-offshore further south, the opposite movement to the east was observed [Figure 4]. The fact that the crustal movements differ according to the region presents complex issues. The research team pointed out that while the cause of the off-Miyagi fluctuations could be explained as “visco-elastic relaxation," for the Fukushima-oki fluctuations, the enduring area at the time of the giant earthquake is currently moving slowly, which can be considered as an “after-slip” occurring subsequently to the great earthquake.

Why is it important to determine what happens after a giant earthquake? This is “because it is important to understand not only the instant at which it occurred but also its continuous cycle before and after the occurrence,” Prof. Kido explains. “Particularly, when a giant earthquake occurs, the ‘clean-up’ movements occur for many years after the earthquake itself. Movements related to the 2011 Tohoku-oki Eq. are continuing even now. The rapid movements seen immediately after the earthquake are weakening now, but it will take decades to return to the state it was in before the earthquake.”

To accurately grasp movements in the earth’s crust after an earthquake, it is vital to understand how strain is released or how strain is further accumulated within the plates; this is extremely important when considering how these factors are connected to earthquakes that may occur in the future. It is not scientifically possible, at present, to accurately predict the date of an occurrence or the magnitude of an earthquake. Nevertheless, this type of elucidation of fluctuations in crustal movements may be important reference material for evaluating the risk of earthquakes and setting up a disaster risk reduction plan.

“Plates near the Japan trench that had not moved since the Jogan Earthquake (869) are thought to have shifted considerably in 2011.” Scientific data of giant earthquakes in a cycle ranging from several hundred to a thousand years did not exist in the past. When considering the frequency of giant earthquakes, unless observation is conducted at the current timing, we will lose the opportunity to gain information for several hundred years in the future. It can be said that the research of Prof. Kido and his team—that elucidates the movement of the seabed, which is not easy to survey, as well as the effects of extremely rare giant earthquakes—is highly significant.

Future research

Phenomena called “slow-slips” are often observed in the area around the occurrence of a giant earthquake. They are intermittent and slow ruptures, emitting no seismic waves, and are so to speak “slow motion earthquakes.” They affect the accumulation and release of strain in the same way as other earthquakes and can also be a trigger for giant earthquakes. In recent years, “slow-slips” have been confirmed around the world, and Prof. Kido and his team plan to continue this research moving forward. Much more frequent observations are required to monitor seafloor displacement due to slow-slips for which movements occur in monthly units. The current method, in which these are measured each time by boarding a vessel and travelling to an area near the trench, limits the frequency of observations and involves a huge cost. Moving forward, Prof. Kido states that he wants to forge ahead using automated measurements with unmanned vessels. Furthermore, despite the fact that giant earthquakes have occurred in the past in the Ryukyu Island and Kuril Island offing, there are regions that do not have observation systems, and extending the offshore geodetic network is a major issue. In the future, Prof. Kido will continue his advance in his exploratory research, elucidating the mechanisms of earthquake occurrence, in the deep sea that is the final frontier.

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