Seafloor Geodesy Transitioning to continuous measurements of plate

Seafloor Geodesy: Transitioning to continuous measurements of plate motion in subduction zones Dave Chadwell (cchadwell@ucsd. edu) Scripps Institution of Oceanography With thanks to: H. Fujimoto, M. Fujita , K. Gagnon , M. Kido, Y. Matsumoto, Y. Osada, K. Tadokoro

Outline 1. 2. 3. 4. 5. 6. 7. Scientific motivation - Need for sea floor meas. Acoustics for Horizontal – Direct, indirect, w/ GPS Pressure for Vertical – Inst. drift, ocean density Bottom moored acoustics – Cont. indirect AUV w/ GPS-Acoustics – Cable installation. Pressure w/ ext. reference– Inst. drift control Buoy for cont. horiz. /vert. - Continuous GPSA, drift controlled press. , water column densit, and sound speed. 8. Summary

Subduction zone geodetics: Limits of land-based GPS Question: Where is the locked zone? Approach: Assume elastic strain = f(convergence rate, fault geometry, lock zone). Observe elastic strain on continent with land-based geodetics, convergence rate from far-field GPS or geology, assume or solve-for thrust fault geometry, and solve for the lock zone: 1. No deformation measurement in the submerged up-dip region. 2. No direct geodetic observation of convergence adjacent to the subduction zone. A need for seafloor geodesy …. .

Acoustic geodesy techniques for cm-level resolution • Electromagnetic waves travel long distances with small uncertainty due to propagation velocity, but do not penetrate significantly sea water. • Acoustic waves travel long distances, but propagation velocity uncertainty limits geodetic usefulness to ~ 10 km. Refraction can be significant. Direct path (0 -2 km) (e. g. , Chadwell et al. , 1999, GRL) Indirect Direct path (2 -10 km) (e. g. , Sweeney et al. , 2005, Marine Geodesy) GPS-surface path (10 -1000 s km) (e. g. , Spiess et al. , 1998, PEPI)

Time/Space resolution of existing (published) horizontal deformation measurement systems

GPS-Acoustic measurements of subduction motion Convergence: Northern Cascadia: (Spiess et al. , 1998, PEPI) Japan Trench: (Osada et al. , 2006) Central Cascadia: (Chadwell et al. , in prep) “non-uniform” Interseismic and Coseimic : Peru: (Gagnon, Chadwell, Norabuena, 2005, Nature) Japan Trench: (Fujita et al. , 2006; Matsumoto et al. , 2006) Nankai Trough: (Kido et al. , 2006; Tadokoro et al. , 2006)

Outline 1. 2. 3. 4. 5. 6. 7. Scientific motivation - Need for sea floor meas. Acoustics for Horizontal – Direct, indirect, w/ GPS Pressure for Vertical – Inst. drift, ocean density Bottom moored acoustics – Cont. indirect AUV w/ GPS-Acoustics – Cable installation. Pressure w/ ext. reference– Inst. drift control Buoy for cont. horiz. /vert. - Continuous GPSA, drift controlled press. , water column densit, and sound speed. 8. Summary

Vertical geodesy techniques for cm-level resolution (e. g. , Phillips et al. , 2008, JGR)

Time/Space resolution of existing (published) vertical deformation measurement systems

Vertical motion from pressure gauges Mid-Ocean Ridge inflation: Axial Volcano: (Chadwick et al. , 2006, J. Vol. Geoth. Res. ) Island collapse: South Flank Kilauea Volcano : (Phillips, Chadwell, Hildebrand, 2008, JGR)

Outline 1. 2. 3. 4. 5. 6. 7. Scientific motivation - Need for sea floor meas. Acoustics for Horizontal – Direct, indirect, w/ GPS Pressure for Vertical – Inst. drift, ocean density Bottom moored acoustics – Cont. indirect AUV w/ GPS-Acoustics – Cable installation. Pressure w/ ext. reference– Inst. drift control Buoy for cont. horiz. /vert. - Continuous GPSA, drift controlled press. , water column densit, and sound speed. 8. Summary

tectonic fault seismic

Continuous (daily) Indirect Path Approach Observations: Each Node interrogates (20 x) all transponders for the two-way travel time Unknowns: Local east, north position of node and east, north and up position of transponder Goal: Daily cm-level resolution (not to scale) Chadwell-SIO/ BP America Inc.

Outline 1. 2. 3. 4. 5. 6. 7. Scientific motivation - Need for sea floor meas. Acoustics for Horizontal – Direct, indirect, w/ GPS Pressure for Vertical – Inst. drift, ocean density Bottom moored acoustics – Cont. indirect AUV w/ GPS-Acoustics – Cable installation. Pressure w/ ext. reference– Inst. drift control Buoy for cont. horiz. /vert. - Continuous GPSA, drift controlled press. , water column densit, and sound speed. 8. Summary

AUV-based GPS-Acoustic • Obs. without research vessel • Obs. With considering sea state and GPS satellite distribution • Immediate response to event • Frequent Obs. M. Mochizuki, A. Asada, T. Ura（Univ. of Tokyo） M. Fujita, M. Sato, Y. Matsumoto （Japan Coast Guard） O. L. Colombo （NASA） T. Tanaka, Z. Hong （SEA Corp. ） K. Nagahashi（MES Co. , Ltd. ）

tectonic fault seismic

Outline 1. 2. 3. 4. 5. 6. 7. Scientific motivation - Need for sea floor meas. Acoustics for Horizontal – Direct, indirect, w/ GPS Pressure for Vertical – Inst. drift, ocean density Bottom moored acoustics – Cont. indirect AUV w/ GPS-Acoustics – Cable installation. Pressure w/ ext. reference– Inst. drift control Buoy for cont. horiz. /vert. - Continuous GPSA, drift controlled press. , water column densit, and sound speed. 8. Summary

Drift control using pressure reference Tryon &Brown capture ambient pressure at deployment, thermally isolated reference; system about to be deployed with buoy. Sasagawa & Zumberge use a “dead” weight that is “raised’’ to preset height by a generated pressure that forms the reference; bench test completed.

Outline 1. 2. 3. 4. 5. 6. 7. Scientific motivation - Need for sea floor meas. Acoustics for Horizontal – Direct, indirect, w/ GPS Pressure for Vertical – Inst. drift, ocean density Bottom moored acoustics – Cont. indirect AUV w/ GPS-Acoustics – Cable installation. Pressure w/ ext. reference– Inst. drift control Buoy for cont. horiz. /vert. - Continuous GPSA, drift controlled press. , water column density, and sound speed. 8. Summary

Continuous GPS-Acoustics / vertical from a moored buoy NS-OTIC 3 -year project started 1/1/07, Brown, Chadwell, Send, Tryon at SIO to build a horizontal (GPSA) and vertical (pressure) sensor. Deployed in August 2008 for initial multi-month test offshore San Diego. Vertical: drift-controlled pressure w/ water density, surface pressure, and surface height measured Horizontal: GPS-Acoustic measurements every 60 seconds.

Seafloor geodetic spatial & temporal resolutions 1. 2. 3. Gray-shaded regions show demonstrated resolutions. Blue-shaded region shows technique fielded for tests. Purple-shaded region shows technique about to be deployed for testing. • Q 1 What will be the temporal threshold of the acoustic and GPS-acoustic systems? • Q 2 Will it be possible to control long-term drift of VSM and can absolute reference surface be realized over 100 s km?

Summary 1) Seafloor geodetic techniques have measured crustal motion with centimeter-level resolution in the offshore portion of subduction zones. 2) Seafloor and sub-aerial geodetic measurements can be combined to span entire oceanic / continental extent of subduction zone. 3) Continuous “daily to sub-daily” observations of vertical and horizontal deformation will be possible during the next decade of subduction zone research.