Development of cabled seafloor seismo- geodetic network and seafloor borehole observatories in the Nankai Trough Eiichiro Araki and Yoshiyuki Kaneda Japan Agency for Marine-Earth Science and Technology (JAMSTEC) Margins SEIZE-WS 08/09/25

Nankai Tonankai Tokai M8+ class earthquakes every years. Sometimes, these areas rupture in a short period of time. Seismogenic Zone Earthquakes in the Nankai Trough 2 Philippine Sea Plate DONET target area

To understand the recurrence patterns in next mega thrust earthquakes Advanced simulation for the estimation of next mega thrust earthquakes New research project

FEM model for Southwest Japan Crust Mantle PHS Thermal 1) Evaluation of heterogeneous structure, thermal structure, etc. 2) Combination of GPS and InSAR data for slip inversion InSAR 1995 Kobe eq. CFF variation Mantle PHS: Slab 3) Inland earthquake model during Nankai earthquake cycle To improve the Simulation Model & Method New research project

When DONET is completed in 2010 Observation site map off Tokai area Tonankai Tokai DONET observation targets: Large earthquakes of M8+ Tsunamis Co-seismic and interseismic ground deformation Microearthquakes Slow earthquakes DONET 5

Historical background: Sumatra earthquake in 2004 drove us to build DONET seafloor network in Nankai. We conducted OBS network observation above the rupture area ONLY 2 months after the earthquake. But we do not know what happed during the 2004 Sumatra earthquake. We need comprehensive seafloor data during the main shock (and interseismic period) to capture behavior seismogenic plate boundary. After Araki et al., 2006.

Location map of DONET seafloor and borehole observatories During the phase I implementation, five science node (Node A-E) will be installed. Five nodes give us connectivity of 40 scientific sensors that are apart from the node up to 10 km (or 20 km optional). 30W (or 50W optional) power can be supplied to each scientific sensor. Precision timing (to a resolution of micro second), and uplink data capability (~20Mbps/sensor) over fiber optic cable are provided to each sensor. 20 seafloor sensors, and three borehole observatories are currently on the list to connect to the DONET network. Red line: DONET cable Pink points: DONET seafloor observatories

Network design to optimize ability to locate small earthquakes Montecarlo simulation test to assess DONET ability to locate small earthquakes Procedures 1.Distribute target earthquakes in grid 2. Compute travel time to observatories for two cases; A)Only land observatories B)Land and DONET observatories 3. Add random observation noise to the computed travel time 4. Locate target earthquakes using these travel time data 5. Evaluate deviation from true location. See percentage of earthquakes located within 5km from the true location. Repeat 100 times for each target earthquake. Procedures 1.Distribute target earthquakes in grid 2. Compute travel time to observatories for two cases; A)Only land observatories B)Land and DONET observatories 3. Add random observation noise to the computed travel time 4. Locate target earthquakes using these travel time data 5. Evaluate deviation from true location. See percentage of earthquakes located within 5km from the true location. Repeat 100 times for each target earthquake.

With DONET, location of earthquakes of M2 can be determined with good accuracy in depth. With DONET, location of earthquakes of M2 can be determined with good accuracy in depth. Hypocenter determined < 5km accuracy Depth determined < 5km accuracy Depth of Initial hypocenter set to actual depth + 20km. Initial hypocenter is distributed randomly Only with land data 9 Network design to optimize ability to locate small earthquakes Land + DONET data

Error assumed in the simulation GPS 1mm (H) 3mm (V) Pressure gauges 1cm(V) Vertical deformation maximum at 3 cm 10 cm Detection of seafloor deformation by DONET pressure gauges With DONET, we will be able to detect small seafloor deformation. This is not possible with land GPS data only.

Design of DONET seafloor sensor Need for wide dynamic range, broad frequency range targeting micro eqs., large eqs., slow slip events, ground deformation. Observation parameters: Ground motion (strong motion accerelometer, broadband seismometer), Pressure (Quartz pressure gauge, differential pressure gauge, hydrophone), Temperature Seismic sensor will be buried ( low noise observation, better coupling)

DONET sensor chooses seismometers and pressure gauges suitable to observe very small to large earthquakes, Tsunamis, and slow ground deformation. We need set of sensors of very broad frequency range and wide dynamic range. Choosing appropriate sensors for target phenomena Slow ground deformation Tsunamis Longperiod Seismic waves Earthquakes Slow ground deformation Longperiod Seismic waves Earthquakes

Concept design of DONET observation sensors. Seismometers are buried in the seafloor and pressure gauges are installed in the seafloor. Mounted on gimbals

Experiments to develop DONET sensor Quartz pressure gauge comparison in laboratory Seismometer comparison in JMA vault. Buried seismometer observation in the seafloor.

Stability test result of quartz pressure gauges Hewlett Packard 2813E, Paroscientific 46K and 410K, and COM quartz pressure gauges). Test pressure is 4000 psi (applied by dead weight tester) and temperature is 4 in water bath. Before May 20, experiment has been interrupted several times. Evaluated long-term stability of quartz pressure gauges less than 0.1 psi drift / 150days

Seismometer evaluation test at Matsushiro vault of JMA. Sensor noise is evaluated for many type seismometers. Broadband seismometer: Streikeisen STS2, Guralp CMG3T, Kinemetrics Cronos Strong motion accerelometer: JAE JA-5typeIII, JA40G, Metrozet TSA-100S Geophones : GS-11D, HS-1, SM-6, L-28 CMG3T STS2 geophones GS-11D, SM-6, HS-11, L28 TSA100S JA5typeIII, JA40G

From results of seismometer evaluation test in Matsushiro Vault (JMA) Wide frequency band Wide dynamic range Combination of Broadband seismometer Guralp CMG3T) and low-noise strong motion accerelometer Metrozet TSA100S) Typical Amplitude of very low frequency earthquakes In the Nankai Trough Combined seismometers for wide frequency and dynamic range

ROV-Homer DPG BBOBS SAM Comparison test observation to evaluate installation method for DONET (on the seafloor, on the seafloor with improved anchor design, and buried sensor) 18m ~150m Buried seismometer Seafloor seismometer Seafloor broadband seismometer with improved anchor design Pressure Temperature Bottom current observation Pressure Dec 07-Feb 08, JAMSTEC NT07-23, NT08-04 cruise

Pressure #1 Pressure #2 Vertical seafloor #1 Vertical seafloor #2 Vertical Buried Horizontal Buried Horizontal seafloor #1 Horizontal seafloor #2 Comparison of buried and seafloor long-period seismic records 2008/1/22 17h second period Vertical noise similar to seafloor seismometer. Horizontal noise is quieter by more than 10 times. Scale of horizontal component is 8 times of vertical Unit in m/s for seismometer and Pa for pressure. Teleseismic earthquake

Seafloor experiment of this year (Sept 17, 2008-late Nov, 2008): Test with proto-type DONET sensor to check installation procedure. Buried sensor (broadband and strong motion sensor inside)

Phase 1.5 : connection of NantroSEIZE borehole observatories Riserless ~ 1km hole below sediment layer. Riserless ~ 500m hole penetrating shallow splay fault Riser ~ 2.5km hole All of these observatories will be connected to DONET Scientific submarine cable network to assure long-term monitoring. The cables will be laid out in Installation of the observatories will take place after Red line: DONET cable Pink points: DONET seafloor observatories Location of the borehole observatories are planned within the reach (10km or 20km) of DONET. Connection of the borehole observatories will be funded as a part of JAMSTECs borehole observatory project (not by DONET).

Seismic structure and models of strain change due to slip in plate boundary. Zone where VLF earthquakes occur.Proposed location of observatories * Land observation has no ability to model where and how strain due to subducting slab is accommodated. * Physical conditions to make plate boundary is not well understood. seismogenic plate boundary

Borehole observation system in the NantroSEIZE Major target: observation of deformation at the seismogenic plate boundary. Why borehole? Low noise environment, better connection to the deformation source (direct measurement of the fault itself in the future). Sensors: Strain (3 + α components) Pore-P (including of splay fault) Tilt, Seismic (broadband, strong motion, geophones) Array of thermometer, electrodes. (Future implementation of continuous active source experiments under consideration). DONET connectivity : Power, data recovery, and precision timing.

Summary DONET network capability: 40 sensors extended 10km (20km) from 5 nodes. 30W power/sensor, precision timing, data communications. The Network covers area from near the trench to Tonankai co-seismic rupture area. 1) Phase 1 (- 2010) - 20 seafloor sensors (BB + strong motion buried seismic sensors + Quartz pressure gauge, DPG, hydrophone, and thermometer) – 30 km spacing between sensors capable of determining precise depth of micro earthquakes and detecting Mw6 class slow events. 2) phase 1.5 (-2011?) - 3 borehole observatories: 2.5km riser hole, 1km, 500m riserless hole with strain, pore-p, tilt, BB+strong motion seismic, thermometers, and electrodes. 3) Phase 2 to extend network westward. (planning) Extended observation items include electromagnetic, tilt, and real-time GPS- Acoustic observation.