Seafloor movements associated with the 2011 Tohoku Earthquake detected by GPS/acoustic geodetic observation Tadashi Ishikawa Hydrographic and Oceanographic Department Japan Coast Guard(JCG) UNAVCO 2012 Science Workshop Mar. 1, 2012 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.)

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.

Outline 1.Objective ~ What to measure ~ 2.Observation System ~ How to measure ~ 3.Observation Results ~ Seafloor movements associated with the Tohoku EQ ~

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

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

Ground station 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) GEONET was established for the monitoring the crustal deformation by GSI (Geospatial Information Authority of Japan) 1 Objective

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

A lot of geodetic data on the ground However 1 Objective

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

Our Seafloor Reference Points 1 Objective Nankai Trough Japan Trench TOHOKU TOKYO Since 2000, The JCG has installed seafloor reference points to monitor the crustal deformation in the sea area TerrestrialSeafloor GEONETSeafloor Reference Point techniqueGPSGPG/Acoustic organizationGSIJCG # of sites~ Depth: m

Outline 1.Objective ~ What to measure ~ 2.Observation System ~ How to measure ~ 3.Observation Results ~ Seafloor movements associated with the Tohoku EQ ~

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

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

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

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

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

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

KGPS analysis GEONET(GSI) Terrestrial Reference Station Mast-top GPS antenna RINEX; 2Hz data sampling Ephemeris; IGS Final orbit Offshore observation ; very long base line (over 100km) Using IT(Interferometric Translocation) software developed by O.L. Colombo(NASA) Ephemeris 2 Observation System Determination of the mast-top antenna position

Acoustic Ranging 10kHz acoustic pulse (ID + Mesurement) Coded with M-sequence code Using Cross-Correlation method ID Mirror-type Transponder at Seafloor Return the signal if ID number is identified ID Hull-mounted Acoustic Transducer Receive Transmit Measurement of the round-trip travel time b/w vessel and transponder 2 Observation System 102ms204ms

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

Outline 1.Objective ~ What to measure ~ 2.Observation System ~ How to measure ~ 3.Observation Results ~ Seafloor movements associated with the Tohoku EQ ~

Our result obtained at Tohoku region 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 before the earthquake after the earthquake at the earthquake 3 Observation Result

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

Observation date after the Tohoku EQ dateKAMNKAMSMYGIMYGWFUKUCHOS Regular Observation (almost 1-3 times per year) Mar. 11, 2011the 2011 Tohoku earthquake Mar Apr Apr Apr MYGW MYGI FUKU KAMN KAMS CHOS # of Acoustic ranging shot Observation error depends on the number of acoustic shot Ordinary: ~5000 shots2-5cm Urgent observation after EQ: 1/8-1/2 of ordinary caseover 10-20cm 3 Observation Result

Coseismic movements associated with the Tohoku EQ 3 Observation Result The seafloor stations (near the epicenter) moved 4-5 times larger than the terrestrial GPS stations. 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. The transition uplift to subsidence toward west may be the key to estimate the area of the source region. Sato et al., Science 2011

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

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

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

Postseismic movement 3 Observation Result from GSI web site Horizontal displacement after the mainshock observed by GPS network (GEONET)

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

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

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

Repeating Earthquake along Nankai Trough 1605 Keicho EQ 1707 Hoei EQ 1854 Ansei Nankai EQ(M8.4) Ansei Tokai EQ(M8.4) 1944 Tonankai EQ(M7.9) 1946 Nankai EQ(M8.0) ???? NANKAI TONANKAI TOKAI Nankai Trough Suruga Trough Philippine Sea plate OSAKA NAGOYA

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

Crustral deformation ( ) 5cm/yr Philippine sea plate Philippine sea plate 3-5cm/yr Eurasian plate Eurasian plate

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.