Chapter 1
The Science of Offshore Earthquakes
Field of Expertise: Marine Geodesy

Motoyuki Kido
Summary
Most researchers did not expect that a magnitude 9 earthquake could occur in the Tohoku
subduction zone owing to the lack of an observation technique for crustal stress accumulation
beneath the seafloor, a key driver of earthquakes. However, rapid development in surveying
techniques of seafloor crustal deformation following the Tohoku earthquake revealed the existenceof various tectonic phenomena associated with megathrust earthquakes. The availability of such anobservational infrastructure will also assist in accurate early warning against seismic shaking andtsunamis in the future.
Keywords: subduction mega-earthquake, slow slip, tsunami, seafloor crustal deformation, seafloor
geodetic network

Introduction
Although earthquakes are found in various tectonic settings, massive magnitude 9 (M9)earthquakes only occur near ocean trenches, where the subduction of an oceanic plate occurs. In
a specific area, the recurrence period is thought to range from 500-1000 years. However, they canrecur every 10-20 years globally, and must therefore be considered as frequent and shareddisaster that threatens humankind in the globalized modern society.

1: Problems Revealed by the Great East Japan Earthquake
What happened?
A large interplate earthquake (M7.3) occurred off the coast of Tohoku, northern Japan, twodays prior to the massive M9.0 mainshock on March 11th 2011, which later became known as theGreat East Japan Earthquake or the 3.11 Tohoku earthquake. The first M7.3 earthquake isretrospectively recognized as a foreshock. When the Tohoku earthquake struck eastern Japan, wewere engaged in preparing an urgent survey on the foreshock in Sendai City, considering its
relationship to the impending so-called ‘off-Miyagi’ earthquake, whose average recurrence periodof 38 years was nearly up. Had the main shock been delayed by one day, we would have sufferedheavily at the port as a result of the tsunami; however, had it been delayed by two days, we might
have already been at sea and had an opportunity to obtain valuable on-board data that would have1

further improved our understanding of the earthquake.
After the Tohoku earthquake, most lifelines stopped, but our lab escaped severe damage
from the tsunami as it was located on a hill. Although onshore surveys were immediately resumed,
offshore surveys were suspended due to the unavailability of research ships. Initial survey resultsfrom across the world showed that a considerable displacement had occurred on a large fault near
the trench, further offshore than a typical off-Miyagi earthquake. However, these results werededuced from indirect evidence, such as tsunami height/inundation, seismic waveform, or onshoregeodetic data, and direct evidence based on seafloor geodesy was not included. Two weeks after
the 3.11 Tohoku earthquake, the first opportunity to conduct a seafloor geodetic survey finallyarose. The offshore surveys revealed that the magnitude of seafloor displacement increased
toward the trench by up to ~30 meters (Figure 1-1), indicating that the interplate fault had moved
several tens of meters. This crucially aided in elucidating the nature of the M9 earthquake.

Figure 1-1. Observed seafloor coseismic displacement associated with the 3.11 earthquake (byTohoku University and Japan Coast Guard)

2: Paradigms Destroyed by the Earthquake
Conventional wisdom and necessary responses
The required observation speed differs greatly between scientific and disaster prevention
purposes. For scientific applications, data accuracy and extent are prioritized over speed.
However, for disaster prevention, real-time observations are necessary for tsunami warning, andeven for risk evaluation of an induced earthquake, a daily, fast operation is most preferable. At thetime of the 3.11 Tohoku earthquake, seafloor geodetic surveys were still early in development, andthe distribution of existing survey sites was sparse. Therefore, we made much account of repeatedregular observations to measure steady strain accumulation for risk evaluation rather than of
emergency surveys, which resulted in a significant time lag prior to the first measurement after theearthquake. The essence of an earthquake is the instantaneous release of accumulated strain that
concurrently generates additional strain in adjacent areas. This is followed by slow relaxation due
to gradually decaying viscous flow and afterslip over several years. It is unfortunate that the datafrom the initial stage of such a process were missing due to the time lag.
From a scientific perspective, sparse and unevenly distributed observation sites could not2

resolve strain accumulation near the trench, where little seismic activity of the interseismic period
had previously been observed, and slow slip activity had not been well understood prior to the 3.11
event. Recently, slow slip events have been highlighted as their propagation can potentially triggera large earthquake (Scientific Research on Innovative Areas “Science of Slow Earthquakes”,2019).

3: A New Approach
Similar to the onshore survey, improving the seafloor geodetic survey is a key task. A GNSS
network (Global Navigation Satellite System, including GPS, Michibiki, etc.), called GEONET,consisting of more than a thousand sites, covers the entire Japanese archipelago and yields
continuous data in real-time. In contrast, only ~20 offshore sites were in operation around Japanprior to the 3.11 event (Kido, 2013) and the number has only marginally increased to ~60 sites
today (Figure 1-2). Furthermore, each offshore site must take a day-long measurement using aresearch ship to obtain stacked data of a single epoch. Therefore, the time resolution (orfrequency) of the data at each site is one month at best, but typically one year at most sites, which
is far from the continuous onshore data. Despite such drawbacks, intensive surveys were repeatedalong the Japan trench after the 3.11 Tohoku earthquake, revealing that the features of postseismic deformation are significantly different in the epicentral off-Miyagi region compared to theadjacent off-Sanriku and off-Fukushima regions. In the Nankai Trough, where a large earthquake ispresumed to occur in the near future, the coupling condition of plate interfaces is evaluated throughobservations that affect tsunami risk. Furthermore, seafloor geodetic sites were expanded into
Ryukyu and Kuril trenches to fill the data gap.
For disaster prevention purposes, numerous seismometers and pressure gauges were installed
and linked through seafloor cabling systems (DONET & S-Net) for real-time data monitoring, likethe onshore network. This plays an essential role in earthquake and tsunami early warning(Network Center for Earthquake, Tsunami and Volcano, n.d.).

Figure 1-2. Present seafloor geodetic sites (GNSS-acoustic survey)

4: Achievements and the Future
A new approach to disaster science

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The rapid development of seafloor geodetic surveys and their infrastructure over the past
decade has been remarkable and has greatly advanced scientific knowledge. However, it has not
yet progressed to the practical stage for emergency operations in disaster prevention andmitigation applications, unlike the cabled real-time systems. This partially stems from budgetaryissues, but mainly from an inverse relationship between the number of sites and survey
frequencies due to the limited ship time and human resources. We are engaged in developing anautonomous and unmanned survey platform, such as a marine drone, to overcome this problem.
We attached our measurement device to a commercially available barebones platform, called a
WaveGlider, and refined the software algorithm for a long-term continuous survey. This system isalready in operation (Figure 1-3). The next steps are to develop an effective data transmission
system and reduce costs to enable the simultaneous deployment of multi-platform operations.
If numerous platforms can be operated simultaneously, we can monitor seafloor
deformation with an incomparably higher frequency than before. This would enable the detection ofslow slips with a time constant of several weeks to months. The permanent operation of such asystem would enable on-demand measurements immediately after a major earthquake or evenduring an earthquake as a finite time event. This is what the onshore or cabled system hasenabled. Furthermore, improvements in real-time data transmission will contribute to tsunami early
warning systems.

Figure 1-3. The sea trial of an unmanned surface platform (WaveGlider).

Conclusion - from the author
Seismology in Japan improved after every devastating earthquake, such as the 1923 Great
Kanto Earthquake, the 1995 Great Hanshin-Awaji Earthquake, the 2011 Great East Japan
earthquake (the 3.11 event). An M9-class earthquake only occurs once every several hundredyears in a particular area; therefore, failure to obtain data during such an event means apermanently lost opportunity to understand the event from a scientific perspective. Hence, we must
utilize the lessons learnt from the 3.11 event, i.e., sufficient preparation for earthquakes in bothdisaster prevention and scientific aspects, through a well-formulated strategy.

References
Kido, M. (2013). Seafloor Geodesy: Its Present State of Affairs and Future Prospect, Journal of theGeodetic Society of Japan, 59(3), 99-109. https://doi.org/10.11366/sokuchi.59.99 (In Japanese)
Network Center for Earthquake, Tsunami and Volcano. (n.d.). Operation of an observation network
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for earthquakes, tsunamis and volcanoes to monitor what is happening every moment across thecountry. https://www.bosai.go.jp/e/research/center/network.html
Scientific Research on Innovative Areas “Science of Slow Earthquakes”. (2019). Science of Slow
Earthquakes. https://www.eri.u-tokyo.ac.jp/project/sloweq/en/newsletters/pdf/leaflet_EN.pdf

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