Installation of a Seafloor Geodesy Observatory in the Northern Chille Subduction Zone

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

The last ten years has seen many magnitude 8-9 subduction zone megathrust earthquakes, several of which generated remarkably destructive tsunami. A majority of fault slip during such events occurs offshore underneath the continental shelf of the overriding plate. Limitations in the state of knowledge before these great earthquakes, and documentation of their rupture process, is a direct result of the lack of offshore seismic and geodetic instrumentation above the fault rupture zones. This study adds to an international effort that will initiate installation of seafloor instrumentation on the Iquique segment of the Nazca-South American plate boundary, on which a strong to devastating (Mw >8.5) earthquake is expected to occur. Installing instrumentation now will allow monitoring of deformation in the terminal stage of a seismic cycle leading up to a great earthquake. The region is unique in that predicted likelihood of its readiness for a major quake appears to be well-founded. A recent M8.1 earthquake just to the north of the study area demonstrates that geodetic coupling models were correct that this segment of the Nazca-South America plate boundary has been locked and built up considerable strain that will continue to be released seismically. A number of mechanical and hydrological properties control whether a given large subduction earthquake will involve high-hazard, shallow slip near the trench or not. One factor that can be quantified in a short-term study is the ongoing deformation of the forearc near the trench, which can be related through numerical models to the locking (versus slowly, aseismically slipping) status of the underlying fault. The onshore/offshore seismic and geodetic observatory to be installed in the Iquique segment through a Chilean-German-US collaboration has potential to be the first interseismic-coseismic-post-seismic dataset for a M>8 earthquake with instrumentation spanning all the way from the trench through the locked zone. The US component of the study will deploy 9 seafloor acoustic ranging instruments along a 15 km transect across the forearc slope for 3 years. Stations will be sited on the basis of new AUV seafloor mapping and will be nested within a ~100 km shelf-trench transect deployed for the main study. The colocation of each group's different acoustic instruments provides an important opportunity to cross calibrate different technology, which can help transition seafloor geodesy out of the past proofing stage into a robust science. Training of a graduate student in acquisition and processing of the geodetic data will increase the workforce in this developing research area.