Seafloor geodetic observation
GNSS-A system

Geodesy & Geophysics Office, Hydrographic Surveys Division,
Hydrographic & Oceanographic Department, Japan Coast Guard
Collaborated with Yokota lab. Institute of Industrial Science, U. TokyoJHOD web siteJune, 2019

Outline

2 GNSS-Acoustic combination techniqueof seafloor geodetic observation
TheJapan
CoastGuard
carriesoutseafloor
geodetic
observation using the “GNSS-Acoustic ranging combination system(GNSS-A)” and measures the seafloor movement around Japan. Thisobservation contributes to earthquake disaster prevention by
providing valuable data for elucidating the mechanism of huge
earthquakes that occur along the Japan Trench and the Nankai Trough.

1 Seafloor geodetic observation

With the spread of GNSS, it has become possible to measure the movement of theground with a precision of 1 cm or less on land. However, GNSS cannot directly
measure the seafloor movement since radio waves are unable to reach theseafloor.
Employing the know-how of seafloor survey and crustal movement observationthat have been cultivated through the production of nautical charts, Japan Coast
Guard has done research and development to measure seafloor crustal movement
in collaboration with the Institute of Industrial Science, University of Tokyo by
combining the radio wave and acoustic techniques❶ using GNSS, determine the survey vessel position（GNSS ranging）❷ using acoustics, measure the range between the vessel and seafloor station
An observation system (GNSS-A) that precisely measures a position on the seafloor
was put to practical use.

Observation networks have been deployed in the focal regions of the 2011 Tohoku
Earthquake and the Nankai Trough Earthquake which is expected to occur in thefuture. Seafloor geodetic observation is conducted regularly in these regions.

40°N

GNSS satellites
JCG sites
Tohoku Univ.

chGNSS ranging

Japan

Tr

en

38°N

36°N

vesselGNSS station

acoustic ranging

acoustics

34°N

N

32°N
132°E

ai
ank

134°E

Tro

136°E

ugh

138°E

seafloor station

140°E

142°E

144°E

JCG seafloor geodetic observation network (June 2018)▎ Geodetic observation

Geodetic observation is performed to understand the phenomenon that occurs under the ground. Thephenomena that occur in relatively shallow places in the Earth have some effect on the Earth's surface, such as
magma movement under the ground which causes the ground to expand, and earthquakes which move the surface.
We observe such ground movement by geodetic observations.

～Island arc
Subducting plate
Outline of GNSS-A system｜ 2

Purpose

Geodetic observation is one of the powerful tools to estimate the distribution of interplate coupling. Amovement that occurs underground affects the surface; simply, the movement of the surface is large if
the continental plate is pulled strongly, and small if the pull is weak. By measuring the movement ofthe surface, the movement underground can be estimated. In Japan, geodetic observations
carried out by the Geospatial Information Authority of Japan (GSI) using an extensive GNSS
network on land are achieving results. However, most of the focal regions of megathrustearthquakes are offshore, making it necessary to observe not only the crustal movement onland but also on seafloor to accurately estimate the distribution of interplate coupling.

of seafloor geodetic observation
The Japan Coast Guard carries out
seafloor geodetic observations at thefloor of the continental plate along theJapan Trench and the Nankai Trough.
The main objectives of these observations
are to estimate the interplate coupling andthe slip distribution during earthquakes,
and to provide these data for elucidation of
the mechanism of earthquakes.

Large ground movement

Island arc

1 Understanding the interplate coupling conditions

Ground moves as the plate ispulled down by the lower plate

Weak coupling
Small ground movement

Weak coupling

trenchIsland arc

Subducting plate▎GNSS

GNSS (Global Navigation Satellite System) is a generic term that refers to a satellite system that precisely measures theposition on the Earth by radio wave communication. There are several systems, each operated by different countries, such as
the GPS (Global Positioning System, USA), GLONASS (GLObal'naya NAvigationnaya Sputnikovaya Sistema, Russia), and theQZSS (Quasi-Zenith Satellite System, Japan).

Trench2 Submission of observation results

Not coupled
CoupledInterplate coupling

Tectonic plates are hard pieces of crust covering the Earth surface. Earthquakes occur frequently atboundaries where the plates collide. Mechanism of earthquakes and the formation of mountains andcontinents can be revealed through the observation of the crustal surface movement.
Megathrust earthquake occurs at a converging plate boundary such as an oceanic trench. Many of the huge
earthquakes exceeding magnitude 8 are known to be this type. Megathrust earthquakes tend to have larger focalregions and magnitudes than inland earthquakes.

～

Subducting plate

Interplate coupling conditions and surface movement
Subducting plate

Continental plate▎T ectonic plates▎Megathrust earthquake

Strong interplate coupling

trenchMegathrust earthquakes occur repeatedly at certain places. This is due to the unevenness ofinterplate coupling at the interface of the continental plate and the subducting oceanic plate.
At regions where the plates arecoupled strongly, the subducting oceanic
plate pulls the continental plate, causing
strain to accumulate in the continentalplate. A huge megathrust earthquake
occurs when the accumulated strain isreleased suddenly. On the other hand,the oceanic plate subducts smoothly
along regions where the interplate
coupling is weak. It is hypothesized thatthe accumulation of the strain in thecontinental plate is smaller in regions
where the interplate coupling is weak. By
accurately determining the distribution of
the strength of interplate coupling, wecan deepen our understanding of
megathrust earthquakes, and promotedisaster prevention
that are more
effective against earthquakes that areexpected to occur in the future.

Strong coupling

The results obtained from seafloor geodetic observation are regularly submitted togovernmental committees related to earthquake disaster prevention. Together with theresults obtained by other organizations, these results are important data for conducting
comprehensive assessment of earthquakes and disaster prevention.
▎Earthquake Research Committee

A committee under the Headquarters for Earthquake Research Promotion that collects, organizes, andanalyzes results of observation, survey, and research related to earthquakes that are achieved by relevant
administrative organizations and universities. Comprehensive evaluation is done according to these results.

▎Coordinating Committee for Earthquake Prediction

A committee that exchanges information of surveys, observations, and research results on earthquake
prediction, and conducts academic studies based on these information.

▎Advisory Council for Evaluation of Earthquakes along the Nankai TroughA council held every month by the Japan Meteorological Agency that evaluates the possibility ofearthquakes across the Nankai Trough region, in conjunction with the Prediction Council for the Area under
Intensified Measures against Earthquake Disaster for the Tokai region.

｜ 4

Outline

1 GNSS antennaeof GNSS-A system
In the GNSS-A system, a variety of instruments are used to accurately
measure the position of the seafloor station (mirror transponder).

Observation instruments
2 Motion sensor

GNSS station※GNSS data on land areprovided by the Geospatial
Information Authority of Japan(GSI).

GNSS data on land
HL 03 Meiyo

1 GNSS antennae
GNSS ranging

GNSS data of vessel

Measure the positionof the GNSS antennae

2 XCTD/XBTlauncher

4 Acoustic
transducer

Measure the position of thetransducer on bottom of the vessel

2 (acoustic velocity observation)

Acoustic ranging

XCTD/XBT launcher

Temperature and salinity

Acoustic signal

Velocity profile analysis

Measure the range ofvessel and seafloor station

Vessel motion data

Round trip travel
time dataRollRound trip travel time

Pitch

Yaw

transmitted
signalreceived
signal4 Acoustic transducer～

2 Motion sensor
Seafloor stations (mirror transponders)

Measure the accurate position｜ 6Details

2 Acoustic velocity measurement

In order to calculate the range between the vessel
and the seafloor station from the measured round-tripof GNSS-A system

1 Acoustic ranging

Since radio waves cannot travel far

Crew working on acoustic ranging

distances underwater, acoustic waves
are used to measure the range between
the survey vessel and the seafloor
station (acoustic ranging). The seafloor
station is an acoustic instrument called
the mirror transponder, which receives
the signal transmitted by the transducer(an instrument that communicatesacoustic signals) installed on bottom of
the vessel. The seafloor station sends
the received signal back to the vessel'stransducer, and the round-trip travel
time of the signal is measured.

travel time, the acoustic velocity profile must be
determined accurately. Since acoustic velocity
varies significantly by water temperature and salinity,
measurements of these vertical profiles are carried out
every few hours during an observation.

Submersing an oceanography instrument from the stern
Transmitted signal

Transducer on bottom of the vessel

Observed acoustic signal（Range measured from round trip travel time）

01

２

Signal received here!

correlation

Received signal pattern

Correlation with thereference signal

３４

0.5
0.0
-0.5

40 msec５（sec）As a result of traveling backand forth few thousand meters of
depth, the intensity of the signal
diminishes and the signal patternis deformed. In order to measure
the round-trip travel time inprecision of few microseconds, thewave shape correlation analysis isemployed. By using this method,fine signals that are hidden in
noisy signals can be identified.

Correlation analysis of the received signal～

Received signal

It is important to determine the underwater acoustic velocity profile, since itchanges significantly in time and position.

Effects of the underwater acoustic velocity｜ 8

3 GNSS ranging on the vessel

The position of the survey vessel, which is an intermediate point that links theGNSS satellites and the seafloor station, is measured with a precision offew centimeters in 0.5 second interval by a high-precision GNSS antennae mounted on

4 Measurement of the seafloor station position

The precise position of the seafloor station is measured by combining the survey vessel
position determined from GNSS ranging, and the range between the transducer and theseafloor station measured by acoustic ranging and acoustic velocity observations.

the mast of the vessel. To determine the position of the transducer on bottom of the vessel
from the position of the GNSS antennae, the inclination data of the vessel is necessary.
Inclination of the vessel due to shaking by waves is expressed as rotations in threedirections, which are constantly measured by a dynamic motion sensor in the vessel.

GNSS antennae

Acoustic velocity ×
round trip travel time ÷ 2

Seafloor station is here!
Determining the position of the seafloor station

Measuring the positions of the GNSS

mast of HL 03 Meiyo

antennae and the acoustic transducer

The inclination of the vessel due
to constant shaking by waves ismeasured by adynamic
motionsensor. Inclination is expressed asrotations in three directions. By
obtaining these rotations, the accurate
position of the acoustic transducer
relative to the GNSS antennae can bedetermined, thus making it possible
to accurately determine the position
wheretheacoustic
signaliscommunicated.

GNSS
antennaeRollinitiated, a pole-like instrument with a GNSS
antennae and a transducer on each end wasused for observation. This pole was held on thestern of the vessel during an observation.

Pitch
The long pole was bent when the vessel left theYaw
Acoustic transducer

Three rotations that express the inclination of the vessel～In the early 2000s when the observations wereNostalgicscenes

GNSS antennae mounted on theport (left picture). The picture on the right is ascene of the routine to hold the pole at the stern.
After 2008, the transducer was installed on thebottom of the vessel, making this routine
unnecessary.

｜ 10

Achievement

2 Interplate coupling estimated from seafloor movements

2.1 Interplate coupling estimated from crustal movements on landof seafloor geodetic observation

1

We can only estimate the interplate coupling conditions for approximately half of theseafloor from crustal movements of land observed by the GNSS network. This is because theSeafloor crustal movement along the Nankai Troughobservation points on land are too far from the plate boundary.
36°N

Japan Coast Guard has been continuously performing GNSS-A observations along theStrong

Nankai Trough, where huge earthquakes are expected to occur. From the results of theobservations, the distribution of interplate coupling has been revealed for the first time.
Weak coupling

Strong coupling

Weak
Cannot be estimateddue to lack of data34°N

Small seafloor movement

Island arc

Estimated interplate
coupling conditions

Large seafloor movement

TrenchTrench

Island arc
Subducting plate

Subducting plate

32°N

36°N

130°E

5.1㎝/yr34°N

5.5㎝/yr
3.9㎝/yr

4.5㎝/yr32°N

4.7㎝/yr

4.8㎝/yr

3.8㎝/yrnNa

4.0㎝/yr

ikao
Tr

134°E

136°E

138°E

2.2 Interplate coupling estimated from seafloor crustal movement
5.0㎝/yr

4.9㎝/yr
3.6㎝/yr
4.3㎝/yr

3.4㎝/yr

132°E

ughInterplate coupling conditions of large areas can be estimated by employing seafloor
crustal movement, as shown in the figure below. The interplate coupling conditions inlarge areas of the Nankai trough are revealed. However, south regions near the troughaxis which are considered to be huge tsunami occurence areas have not been observed yet.
36°N

Estimated interplate
coupling conditions
Strong5～7cm/yr

Weak
Cannot be estimateddue to lack of data

4.2㎝/yr
2.0㎝/yr

130°E
134°E
The GNSS (GEONET) 132°E
observations on land (GSI, blackarrows) and the subduction rate of the Philippine Sea
plate (DeMets et al. 2010, yellow arrow) are shown forcomparison.

Movement of the subducting (Philippine Sea) plate

136°E

138°E
Subducting plate

Y. Yokota, T. Ishikawa, S. Watanabe, T. Tashiro and A. Asada (2016), Nature, doi:10.1038/nature17632.

～34°N

32°N

130°E

132°E

134°E

136°E

138°E

｜ 12

3 Seafloor movements before and after the Tohoku earthquake

3.1 Before the Tohoku earthquake

40°N

The seafloor movements indicate interplate
coupling conditions at the plate boundary.

4.3 cm/yr
3.2 cm/yr

4.2 cm/yr

4.6 cm/yr

Tr
ench

38°N

8～

2.3 cm/yr

Subducting plate

36°N
139°E

3.2 During the Tohoku earthquake

141°E

143°E

145°E

15ｍ
２3ｍ

5ｍ

ch38°N

3.3 After the Tohoku earthquake

36°N
139°E

141°E

Ja

5ｍ

Trench２４ｍ

panT

ren15ｍSubducting plateyr

40°N

A huge seafloor movement of 24 m wasobserved on the seafloor. The difference inmovement indicates that the epicenter islocated offshore in Miyagi.

Island arc

m/Ja

Island arc

9c

pan

Trench

143°E

145°E

40°N

Complicated movements of the seafloor
continue after the earthquake. The difference inmovement is thought to be caused not only by theplates but also by the mantle.

24 cm55 cm
64 cm

38°N

TrenchSubducting plate

80 cm

36°N
139°E

141°E

Ja

Island arcpanTrench

22 cm

143°E

145°E

M. Sato, T. Ishikawa, N. Ujihara, S. Yoshida, M. Fujita, M. Mochizuki, and A. Asada (2011),
Science, doi:10.1126/science.1207401.
S. Watanabe, M. Sato, M. Fujita, T. Ishikawa, Y. Yokota, N. Ujihara, and A. Asada (2014),
GRL, doi:10.1002/2014GL061134.

～海上保安庁 Japan Coast Guard｜ 14