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Tadashi Ishikawa Hydrographic and Oceanographic Department Japan Coast Guard(JCG)

S eafloor movements associated with the 2011 Tohoku Earthquake detected by GPS/acoustic geodetic observation. Mariko Sato, Naoto Ujihara , Shun- ichi Watanabe(JCG) Akira Asada, Masashi Mochizuki (Univ. of Tokyo) Hiromi Fujimoto, Motoyuki Kido (Tohoku Univ.) Keiichi Tadokoro (Nagoya Univ.).

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Tadashi Ishikawa Hydrographic and Oceanographic Department Japan Coast Guard(JCG)

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  1. Seafloor movements associated with the 2011 Tohoku Earthquake detected by GPS/acoustic geodetic observation Mariko Sato, Naoto Ujihara, Shun-ichi Watanabe(JCG) Akira Asada, Masashi Mochizuki (Univ. of Tokyo) Hiromi Fujimoto, Motoyuki Kido (Tohoku Univ.) Keiichi Tadokoro (Nagoya Univ.) Tadashi Ishikawa Hydrographic and Oceanographic Department Japan Coast Guard(JCG) UNAVCO 2012 Science Workshop Mar. 1, 2012

  2. Outline • JCG 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.

  3. Outline • Objective ~ What to measure ~ • Observation System ~ How to measure ~ • Observation Results • ~ Seafloor movements associated with the Tohoku EQ ~

  4. Earthquake distribution around JAPAN North American plate Hypocenter distribution (1998-2007, M>4) Eurasian plate Japan Trench Pacific plate 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. Nankai Trough Philippine Sea plate depth of hypocenter 1 Objective

  5. Mechanism of plate boundary type earthquake North American plate Mechanism of Earthquakes Eurasian plate 8-9cm/yr Pacific plate Crustal deformation data is the one of the most important information to investigate the interplate coupling 3-5cm/yr Philippine Sea plate 1 Objective

  6. Groundstation for monitoring Crustal deformaition(GPS,SLR,VLBI) ・MEXT ・Universities ・NIED ・GSI ・JCG ・AIST GEONET Dense GPS network over 1200 sites GEONET was established for the monitoring the crustal deformation by GSI (Geospatial Information Authority of Japan) 1 Objective

  7. Crustal deformation detected by GEONET (1996-1999) GEONET revealed many interesting geodynamic phenomena relating to the plate motion, mechanism of earthquakes and volcanic activity. reference point GEONET GPS station 1 Objective

  8. A lot of geodetic data on the ground However 1 Objective

  9. ・MEXT ・Universities ・NIED ・GSI ・JCG ・AIST Focal regions of Huge Earthquake lie beneath the seafloor TOHOKU TOKAI TONANKAI NANKAI Lack of data in the sea area limits the investigation of geodynamic phenomena 1 Objective

  10. Our Seafloor Reference Points Since 2000, The JCG has installed seafloor reference points to monitor the crustal deformation in thesea area TOHOKU Japan Trench TOKYO Depth:1000-3000m Nankai Trough 1 Objective

  11. Outline • Objective ~ What to measure ~ • Observation System ~ How to measure ~ • Observation Results • ~ Seafloor movements associated with the Tohoku EQ ~

  12. How to measure the seafloor movement ? Terrestrial Precise measurements using Electromagnetic waves (GPS, SLR, VLBI, ....) Undersea Cannot use Electromagnetic wave due to absorption in seawater Measurements using Acoustic Wave GPS/Acoustic Combination Technique The idea is based on early works by SIO (e.g. Spiess, 1985) 2 Observation System

  13. GPS/Acoustic Combination Geodetic Observation The combination of GPS Positioning and Acoustic Ranging enables seafloor positioning Kinematic GPS Positioning Terrestrial GPS stations Survey vessel Acoustic Ranging Seafloor stations (acoustic transponder) Continental Plate Oceanic Plate Plate boundary 2 Observation System

  14. System Configuration Kinematic GPS Positioning To determine the position of the on-board GPS antenna Acoustic Ranging To measure the travel time b/w the on-board transducer and seafloor transponder Seafloor Positioning To determine the position of the seafloor transponders with cm-level accuracy 2 Observation System

  15. On-board Unit GPS Antenna (Trimble Zephyr Geodetic) Fiber Optic Gyroscope (IXSEA PHINS) Survey vessel "MEIYO" Acoustic Transducer (Link-Quest custom) 2 Observation System

  16. Seafloor Unit Depth （（（））） （（（））） One reference point consists of four acoustic transponder （（（））） （（（））） Transponders are installed at seafloor by a free fall Photo by JAMSTEC 2 Observation System

  17. Flow of data analysis Fujita et al., EPS 2006 Round-trip travel time b/w transducer and transponder Acoustic Wave Data Sound Speed Data Acoustic signal analysis GPS Data Attitude Data Underwater positioning Kinematic GPS analysis Antenna position Transducer position Convert Seafloor Transponder position 2 Observation System

  18. ID ID ID ID Acoustic Ranging Measurement of the round-trip travel time b/w vessel and transponder Hull-mounted Acoustic Transducer • 10kHz acoustic pulse (ID + Mesurement) • Coded with M-sequence code • Using Cross-Correlation method 102ms 204ms Mirror-type Transponder at Seafloor Return the signal if ID number is identified Receive Transmit 2 Observation System

  19. Underwater positioning Transducer position determined by KGPS analysis Distance b/w Transducer and Transponder Acoustic travel time determined by signal analysis Sound Speed obtained by CTD and XBT Determine Transponder Position with cm precision 2 Observation System

  20. Outline • Objective ~ What to measure ~ • Observation System ~ How to measure ~ • Observation Results • ~ Seafloor movements associated with the Tohoku EQ ~

  21. Our result obtained at Tohoku region before the earthquake • Crustal deformation (interseismic period) caused by subduction of the Pacific plate • Coseismic movement caused by the 2011 Tohoku earthquake • Postseismic movement after the 2011 Tohoku earthquake at the earthquake after the earthquake 3 Observation Result

  22. Crustral movement caused by the subduction of the Pacific plate before the Tohoku Earthquake North American plate Miyagi 5.5cm/yr 1.9cm/yr Fukushima Pacific plate 8~9cm/year Tokyo • The seafloor stations moved toward west 2-6 cm per year. • Off Fukushima region is slower than off Miyagi region. 3 Observation Result

  23. KAMN Observation date after the Tohoku EQ KAMS MYGW MYGI FUKU # of Acoustic ranging shot CHOS Observation error depends on the number of acoustic shot Ordinary: ~5000 shots 2-5cm Urgent observation after EQ: 1/8-1/2 of ordinary case over 10-20cm 3 Observation Result

  24. Coseismic movements associated with the Tohoku EQ Sato et al., Science 2011 The seafloor stations (near the epicenter) moved 4-5 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. 3 Observation Result

  25. Result of Tohoku Univ. (GJT3 & GJT4) Kido et al., GRL 2011 GJT4 GJT3

  26. Estimated coseismic slip from geodetic data by GSI Estimated by terrestrial GPS data and seafloor GPS/Acoustic data Estimated by terrestrial GPS data only Maximum 27m Maximum 56m from GSI web site Ozawa et al., Nature 2011 3 Observation Result

  27. Estimated by terrestrial GPS data and seafloor GPS/Acoustic data Estimated by Tsunami waveform inversion Fujii et al., EPS 2011 Maximum 56m >40m 3 Observation Result

  28. Postseismic movement Horizontal displacementafter the mainshock observed by GPS network (GEONET) from GSI web site 3 Observation Result

  29. Postseismic movement MYGI Understanding of this result is the subject of future investigation 3 Observation Result

  30. Future Outlook Reinforcement of the seafloor observation network • Tohoku region • Tohoku Univ. and Nagoya Univ. are planning to install 20 new GPS/A sites • Nankai region • JCG has installed 8 new GPS/A sites in Jan. 2012

  31. provisional plan Tohoku Univ. & Nagoya Univ. • Planned GPS/A sites(~20) • Compatible with JCG system • Main objectives: • ・Afterslip distribution • ・Coupling near the trench New site Existing site (JCG & Tohoku Univ.)

  32. Repeating Earthquake along Nankai Trough OSAKA NAGOYA NANKAI TOKAI 684 887 1099 1361 1498 TONANKAI Suruga Trough Nankai Trough 102 Philippine Sea plate 147 92

  33. Reinforcement of observation along Nankai Trough Existing station New station TOKAI Nagoya Univ. Tohoku Univ. TONANKAI NANKAI Jan. 2012 The JCG installed eight new stations along Nankai Trough

  34. Crustral deformation (2006-2011) Eurasian plate 4cm/yr 5cm/yr 2cm/yr 3cm/yr 4cm/yr 3cm/yr 3-5cm/yr Philippine sea plate 5cm/yr

  35. Summary • We 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.

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