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Maksymowicz, A, Chadwell CD, Ruiz J, Trehu AM, Contreras-Reyes E, Weinrebe W, Diaz-Naveas J, Gibson JC, Lonsdale P, Tryon MD.  2017.  Coseismic seafloor deformation in the trench region during the Mw8.8 Maule megathrust earthquake. Scientific Reports. 7   10.1038/srep45918   AbstractWebsite

The M-w 8.8 megathrust earthquake that occurred on 27 February 2010 offshore the Maule region of central Chile triggered a destructive tsunami. Whether the earthquake rupture extended to the shallow part of the plate boundary near the trench remains controversial. The up-dip limit of rupture during large subduction zone earthquakes has important implications for tsunami generation and for the rheological behavior of the sedimentary prism in accretionary margins. However, in general, the slip models derived from tsunami wave modeling and seismological data are poorly constrained by direct seafloor geodetic observations. We difference swath bathymetric data acquired across the trench in 2008, 2011 and 2012 and find similar to 3-5 m of uplift of the seafloor landward of the deformation front, at the eastern edge of the trench. Modeling suggests this is compatible with slip extending seaward, at least, to within similar to 6 km of the deformation front. After the M-w 9.0 Tohoku-oki earthquake, this result for the Maule earthquake represents only the second time that repeated bathymetric data has been used to detect the deformation following megathrust earthquakes, providing methodological guidelines for this relatively inexpensive way of obtaining seafloor geodetic data across subduction zone.

Chadwell, CD.  2003.  Shipboard towers for Global Positioning System antennas. Ocean Engineering. 30:1467-1487.   10.1016/s0029-8018(02)00141-5   AbstractWebsite

Two 12.2 m-high towers for mounting Global Positioning System (GPS) receiver antennas were designed and constructed to provide millimeter-level stability while maintaining portability and accessibility to satellites and deck spaces. A combination of guys and a 3-m horizontal strut provide roll and pitch stability of 2-3 rum observed from 0.1 seconds to 12 days using a combination of GPS and optical/laser devices. The shipboard antenna mounts connect sub-aerial GPS positioning to underwater acoustic ranging that determine the centimeter-level location of seafloor transponders. Observed annually, these seafloor geodetic positions measure seafloor crustal motion for geophysical studies. (C) 2003 Elsevier Science Ltd. All rights reserved.

Spiess, FN, Chadwell CD, Hildebrand JA, Young LE, Purcell GH, Dragert H.  1998.  Precise GPS/Acoustic positioning of seafloor reference points for tectonic studies. Physics of the Earth and Planetary Interiors. 108:101-112.   10.1016/s0031-9201(98)00089-2   AbstractWebsite

Global networks for crustal strain measurement provide important constraints for studies of tectonic plate motion and deformation. To date, crustal strain measurements have been possible only in terrestrial settings: on continental plates and island sites within oceanic plates. We report the development of technology for horizontal crustal motion determination at seafloor sites, allowing oceanic plates to be monitored where islands are not available. Seafloor crustal monitoring is an important component of global strain measurement because about 70% of the Earth's surface is covered by water, and this region contains most of the tectonic plate boundaries and zones of crustal deformation. Using the Global Positioning System (GPS) satellites and underwater acoustics, we have established a geodetic reference site on the Juan de Fuca plate at 2.6 km depth, approximately 150 km off the northwest coast of North America. We measure the baselines between this site and two terrestrial GPS stations on Vancouver Island, British Columbia. The Juan de Fuca plate site is an appropriate setting to develop seafloor observation methods, since it is a well studied area, easily accessible from west coast Canadian and United States ports. Determination of seafloor motion at this site addresses questions related to convergence between the Juan de Fuca and North American plates across the Cascadia Subduction Zone. At the Juan de Fuca seafloor geodetic reference site, we installed precision acoustic transponders on the seafloor, and measured ranges to them from a sound source at a surface platform (ship or buoy), The platform is equipped with a set of three GPS antennas allowing determination of the sound source position at times of signal transmission and reception. Merging the satellite and acoustic data allows determination of the transponder network location in global reference frame coordinates. Data processing to date suggests repeatabilities of +/-0.8 cm north and +/- 3.9 cm east in the seafloor transponder network position relative to reference points on Vancouver Island. (C) 1998 Elsevier Science B.V. All rights reserved.