Seafloor movements associated with the 2011 Tohoku Earthquake detected by GPS/acoustic geodetic observationTadashi IshikawaHydrographic and Oceanographic DepartmentJapan Coast Guard(JCG)UNAVCO 2012 Science WorkshopMar. 1, 2012Mariko Sato, Naoto Ujihara, Shun-ichi Watanabe(JCG)Akira Asada, Masashi Mochizuki (Univ. of Tokyo)Hiromi Fujimoto, Motoyuki Kido (Tohoku Univ.)Keiichi Tadokoro (Nagoya Univ.)0Today I will introduce our work, seafloor geodetic observation..

Collaborators are here.OutlineJCG has been a developing a system for precise seafloor geodetic observation with the GPS/Acoustic combination technique.The primary purpose is to detect the seafloor crustral movement caused by the subduction of the oceanic plate.JCG have succeeded in detecting seafloor movements caused by the 2011 Tohoku earthquake.

JCG has ..............................I would like to talk to you about brief overview of observation technique and the results of our observation.1OutlineObjective ~ What to measure ~Observation System ~ How to measure ~Observation Results ~ Seafloor movements associated with the Tohoku EQ ~

I divided my presentation into three parts.First is the objective, second is the methodology and finally Id like to introduce results related to the Tohoku earthquake.First, I would like to talk about the objective of observation.

2Earthquake distribution around JAPAN

Eurasian platePacificplatePhilippine SeaplateNorth Americanplatedepth of hypocenterHypocenter distribution (1998-2007, M>4)Japan has historically suffered damage from huge earthquakes.The focal regions of such huge earthquakes usually lie beneath the seafloor, especially on the side of the Pacific Ocean.

Japan TrenchNankai Trough1 ObjectiveAs you know, Japan has historically suffered from damage caused by huge earthquakes.

As you can see, Japan is located in a tectonically active region. Four tectonic plates interact with each other around Japan. Many of focal regions of huge earthquakes are lie beneath the seafloor, especially on the side of the Pacific Ocean. 3Mechanism of plate boundary type earthquakeMechanism of Earthquakes1 Objective

Eurasian platePacificplatePhilippine SeaplateNorth Americanplate8-9cm/yr3-5cm/yrCrustal deformation data is the one of the most important information to investigate the interplate coupling

As you might know, the principle of a plate boundary type earthquake is shown in this figure.

For the research of focal regions of huge earthquakes, observation of crustal deformation gives us the most important information ..4Ground station for monitoring Crustal deformaition (GPS,SLR,VLBI)

MEXTUniversitiesNIEDGSIJCGAISTGEONETDense GPS network over 1200 sitesGEONET was established for the monitoring the crustal deformation by GSI (Geospatial Information Authority of Japan)1 Objective

For that reason. A lot of ground station is now available in Japan. Especially dense GPS network called GEONET was established by GSI .GEONET consists of over 1200 GPS stations.5Crustal deformation detected by GEONET (1996-1999)

GEONET GPS station reference pointGEONET revealed many interesting geodynamic phenomena relating to the plate motion, mechanism ofearthquakes and volcanic activity.1 ObjectiveI'd like to introduce an example of result of GEONET.This figure shows crustal deformation detected by GEONET.

In this way, GEONET revealed many interesting geodynamic phenomena.

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A lot of geodetic data on the ground

However1 ObjectiveLike this 7

MEXTUniversitiesNIEDGSIJCGAISTTOHOKUTOKAITONANKAINANKAIFocal regions of Huge Earthquake lie beneath the seafloor Lack of data in the sea area limits the investigation of geodynamic phenomena1 ObjectiveThe focal regions of huge earthquake lie under the sea and GPS is not applicable on the seafloor.So, lack of data in the sea area limits the investigation of geodynamic phenomena.

To complement the lacked information, we want to measure the movement of the seafloor directly.8

Our Seafloor Reference Points1 ObjectiveNankai TroughJapan TrenchTOHOKUTOKYOSince 2000, The JCG has installed seafloor reference points to monitor the crustal deformation in the sea area TerrestrialSeafloorGEONETSeafloor Reference PointtechniqueGPSGPG/AcousticorganizationGSIJCG# of sites~120027

Depth:1000-3000mSince 2000, We have installed seafloor reference points to monitor the crustal deformation in the sea area. Our seafloor reference points are located as shown in this figure. At present, we have 27 reference points.Reference points are located on the landward slope of the major trench,and are primarily targeting at the focal regions of the plate boundary type earthquakes.9OutlineObjective ~ What to measure ~Observation System ~ How to measure ~Observation Results ~ Seafloor movements associated with the Tohoku EQ ~

Next, I'd like to talk about brief overview of observation system.10TerrestrialPrecise measurements using Electromagnetic waves (GPS, SLR, VLBI, ....)UnderseaCannot use Electromagnetic wave due to absorption in seawater Measurements using Acoustic WaveHow to measure the seafloor movement ?The idea is based on early works by SIO (e.g. Spiess, 1985)GPS/Acoustic Combination Technique2 Observation SystemThe question is "How to measure the seafloor movement ?"

In the case of ground station, it is possible to measure crustal movement precisely by using electromagnetic waves.But in the case of undersea, it is not possible. Because EM wave is absorbed into seawater.

So we use acoustic wave for measurement undersea.

Combination of two techniques enables seafloor positioning.An original idea of GPS/A is based on early works by scientists of SIO.

11GPS/Acoustic Combination Geodetic Observation2 Observation System

Kinematic GPS PositioningOceanic PlatePlate boundaryTerrestrial GPS stationsAcoustic RangingSeafloor stations(acoustic transponder)Survey vesselContinental PlateThe combination of GPS Positioning and Acoustic Ranging enables seafloor positioningThis picture shows our system.

Our observation is carried out aboard a survey vessel on a campaign basis.Frequency of observation is almost 3 times per year at one reference point.Developing a semi-realtime system is the subject to future.12

System Configuration2 Observation SystemKinematic GPS PositioningAcoustic RangingSeafloor PositioningTo determine the position of the on-board GPS antennaTo measure the travel time b/w the on-board transducer and seafloor transponderTo determine the position of the seafloor transponders with cm-level accuracyThe position of the vessel is precisely determined by kinematic GPS. The vessel measures the distance to the seafloor stations using acoustic wave. To combine the two kinds of observation, we get the precise position of the seafloor stations.

This system mainly consists of a seafloor unit and an on-board unit.XBT and CTD are also performed during the observation to get sound speed structure of the seawater.

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On-board Unit

GPS Antenna(Trimble Zephyr Geodetic)Acoustic Transducer(Link-Quest custom)

Fiber Optic Gyroscope(IXSEA PHINS)Survey vessel "MEIYO"2 Observation SystemThis is a photos of shipboard equipment.Our unit consists of mainly three units.14Seafloor Unit

Depth

Transponders are installed at seafloor by a free fallPhoto by JAMSTECOne reference point consists of four acoustic transponder2 Observation SystemSeafloor unit consists of four acoustic transponders.This is photo when we install the transponder.Transponders are installed at seafloor by a free fall.

This photo shows transponder at seafloor.This photo was taken by ROV.As you can see from this photo, lower part of transponder is buried in the mud.So transponder is steady at seafloor.

15Flow of data analysisSeafloor Transponder positionRound-trip travel timeb/w transducer and transponder Acoustic signal analysisAcoustic Wave DataSound Speed DataUnderwater positioningAntenna positionKinematic GPS analysisTransducer positionGPS DataAttitude DataConvert

Fujita et al., EPS 20062 Observation System

Next I will explain the flow of data analysis.

Collected data are four kinds: Acoustic wave, GPS, attitude of the ship and sound speed.By analyzing acoustic wave signal, we get round-trip travel time between ship and transponder.

GPS data is used for determining antenna position from KGPS analysis. The antenna position is converted to the position of the hull-mounted acoustic transducer.Combining these results with sound speed, the precise position of seafloor transponder is estimated.

16KGPS analysis

GEONET(GSI)Terrestrial Reference StationMast-top GPS antennaRINEX; 2Hz data samplingEphemeris; IGS Final orbitOffshore observation ; very long base line (over 100km)

Using IT(Interferometric Translocation) software developed by O.L. Colombo(NASA)

Ephemeris

2 Observation SystemDetermination of the mast-top antenna positionThis is a summary of KGPS analysis.Positions of the GPS antenna are determined by kinematic GPS analysisWe use 2Hz sampled RINEX data and IGS final orbit.

We use the software called IT (for Interferometric Translocation) developed by Dr. Colombo.It is the software for the precise determination of the trajectory of a rover over very long baselines17Acoustic Ranging10kHz acoustic pulse (ID + Mesurement)Coded with M-sequence codeUsing Cross-Correlation methodIDIDMirror-type Transponder at SeafloorReturn the signal if ID number is identifiedIDID

Hull-mounted Acoustic TransducerReceiveTransmitMeasurement of the round-trip travel time b/w vessel and transponder2 Observation System102ms204msThis is a summary of acoustic ranging.The seafloor transponder has a functions to re-transmit a signal .It receives a signal from the vessel and then transmits the signal back.

We measure round-trip travel times b/w hull-mounted transducer and seafloor transponder.

The ranging signal is a 10 kHz acoustic wave, coded with a M-sequence.The length of the whole signal amounts to 102ms and 204 ms. 18Underwater positioning

Transducer position determined by KGPS analysisAcoustic travel timedetermined by signal analysis Sound Speedobtained by CTD and XBTDetermine Transponder Positionwith cm precisionDistance b/wTransducer and Transponder 2 Observation SystemThis is a basic principle of underwater positioning.We get the positions of transducer from KGPS.By multiplying sound speed, acoustic travel time is converted to distance between hull-mounted transducer and seafloor transponder.We collect many data around the transponder. And using a least square method, we can get the seafloor transponder position.

This is a basic principle of our analysis.19OutlineObjective ~ What to measure ~Observation System ~ How to measure ~Observation Results ~ Seafloor movements associated with the Tohoku EQ ~

Next, I would like to introduce results of our observation associated with the Tohoku earthquake.20Our result obtained at Tohoku regionCrustal deformation (interseismic period) caused by subduction of the Pacific plate

Coseismic movement caused by the 2011 Tohoku earthquake

Postseismic movement after the 2011 Tohoku earthquakebefore the earthquakeafter the earthquakeat the earthquake3 Observation ResultWe have three types of results at Tohoku region.21

3 Observation ResultFukushimaMiyagi5.5cm/yr1.9cm/yrPacific plate8~9cm/yearThe seafloor stations moved toward west 2-6 cm per year.Off Fukushima region is slower than off Miyagi region.North AmericanplateCrustral movement caused by the subduction of the Pacific platebefore the Tohoku Earthquake

TokyoFirst, I'll introduce the result before the earthquake.

We have been carried out campaign observation for about 10 year in this region.This figure shows the crustal motion at interseismic period.

From our result, the seafloor stations moved toward west 2-6 cm per year. The black allows shows the velocity of the GPS station.

The crustal velocity at off-Miyagi is fast and close to the subduction velocity of the Pacific plate.In contrast, the crustal velocity of off-Fukushima is significantly slow.The contrast of two results infers the difference of strength of interplate coupling between two regions. 22Observation date after the Tohoku EQdateKAMNKAMSMYGIMYGWFUKUCHOS2000-2011Regular Observation (almost 1-3 times per year)Mar. 11, 2011the 2011 Tohoku earthquakeMar. 27-2920122344616Apr. 3-514211310Apr. 12-1327172424Apr. 184779

MYGWMYGIFUKUKAMNKAMSCHOS# of Acoustic ranging shot Observation error depends on the number of acoustic shotOrdinary: ~5000 shots2-5cmUrgent observation after EQ: 1/8-1/2 of ordinary caseover 10-20cm3 Observation ResultNext, I show the coseismic movements associated with the Tohoku earthquake. This table shows record of observation after earthquake.

We have carried out urgent observation after 2-4 week of earthquake.Our observation has error depending on the number of acoustic shots.Ordinary, we get about 5,000 acoustic shots for one observation epoch.But we got only one over the eight to half of ordinary case.Therefore, error may exceed 10-20 cm.

23Coseismic movements associated with the Tohoku EQ3 Observation Result

The seafloor stations (near the epicenter) moved4-5 times larger than the terrestrial GPS stations.The transition uplift to subsidence toward west may be the key to estimate the area of the source region.Sato et al., Science 2011We detected very large, as large as 24 meter motion, just above the epicenter! The seafloor stations moved 4-5 times larger than the on-land stations.

As you can see, from vertical motion of these three points. The transition uplift to subsidence toward west are shown.It meight be the important information to estimate the area of the source region. 24Result of Tohoku Univ. (GJT3 & GJT4)

GJT4GJT3Kido et al., GRL 2011In addition to our result, I introduce a result of other group of Japan.

This is the result detected at sites of Tohoku University.They also detect large seafloor movement by GPS/Acoustic technique.The maximum of movement is 31m.This is consistent with our result.25Estimated coseismic slip from geodetic data by GSI

Estimated by terrestrial GPS data onlyEstimated by terrestrial GPS data and seafloor GPS/Acoustic data Maximum 27mMaximum 56mfrom GSI web site3 Observation ResultOzawa et al., Nature 2011Next, I'd like to speak what this results show.

Coseismic slip on plate boundary could be estimated using crustal movement data.These results are calculated by GSI.Left figure shows a result using terrestrial GPS data only. This results suggests that maximum of 27m slip occurred near epicenter.

On the other hand using our GPS/Acoustic data in addition to GPS data, maximum slip exceeds 50m. 26

Estimated by Tsunami waveform inversion

>40m3 Observation ResultFujii et al., EPS 2011Estimated by terrestrial GPS data and seafloor GPS/Acoustic data Maximum 56mIn other study, not geodetic method, Fujii et al. 2011 estimated slip from Tsunami waveform inversion method.They showed the result that over 40m slip occurred near the axis of the Japan trench.This study also suggest existence of large slip and comparable with right figure.Such large slip may causes large Tsunami.

This large slip cannot be estimated from ground geodetic data only.Therefore, we can say, the observation at sea area is definitely important to understand the earthquake.27Postseismic movement 3 Observation Result

from GSI web siteHorizontal displacement after the mainshockobserved by GPS network (GEONET)I'd like to shift the subject of postseismic movement.

The result of southern part shows movement toward east-south-east.This movement is consistent with the result of GPS stations.

On the other hand, this west direction movement seems to be somewhat strange.

28Postseismic movement 3 Observation Result

Understanding of this result is the subject of future investigation

MYGIWe do not have enough data to understand this result.Understanding of this result is the subject of future investigation.So we will continue observation, and store the data.29Future OutlookReinforcement of the seafloor observation network

Tohoku regionTohoku Univ. and Nagoya Univ. are planning to install 20 new GPS/A sites

Nankai regionJCG has installed 8 new GPS/A sites in Jan. 2012Finally, I'd like to speak about future outlook.

Japanese government(scientists) are planning to enforce seafloor observation.

In Tohoku region, Tohoku Univ. and Nagoya Univ. are planning to install 20 new GPS/A sites.It will be done this summer.

In Nankai region, we has installed 8 new GPS/A sites in this January.

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Planned GPS/A sites(~20)Compatible with JCG systemMain objectives: Afterslip distribution Coupling near the trenchprovisional planNew siteExisting site(JCG & Tohoku Univ.)Tohoku Univ. & Nagoya Univ.This is the provisional plan of Tohoku and Nagoya university.

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Repeating Earthquake along Nankai Trough

1605Keicho EQ1707Hoei EQ1854Ansei Nankai EQ(M8.4) Ansei Tokai EQ(M8.4)1944Tonankai EQ(M7.9)1946Nankai EQ(M8.0)????10214792NANKAITONANKAITOKAINankai TroughSuruga Trough684887109913611498Philippine Sea plateOSAKANAGOYAId like to move on to the next important area of our interest.Not only Tohoku region, Nankai trough is also the danger seismic zone. The Philippine sea plate subducts beneaththe continental plate.

Up to present huge earthquake has occurred every 100-150 years.Therefore observation in this is are is also important.32Reinforcement of observation along Nankai Trough

Existing stationNew stationNagoya Univ.Tohoku Univ.TOKAITONANKAINANKAIJan. 2012The JCG installed eight new stations along Nankai TroughWe decided to enforce the observation along Nankai Trough.So we have completed to install eight new stations in this January.

33Crustral deformation (2006-2011)

5cm/yr3cm/yr4cm/yr2cm/yr3cm/yr4cm/yr5cm/yrPhilippine seaplate3-5cm/yrEurasianplateThis is the result obtained at existing site.I think that a result at new sites will be obtained in the next 3-4 years. 34SummaryWe have been carrying out GPS/Acoustic seafloor geodetic observations on the landward of the major trenches in the Pacific Ocean.

We detected seafloor movement associated with the Tohoku earthquake. This results will lead to more precise estimation of the fault slip.

Seafloor geodetic observation gives fruitful knowledge about subduction-zone earthquakes.

To conclude, I would like to summarize my presentation.

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