Hikurangi Trench Regional Electromagnetic Survey to Image the Subduction Thrust

DESCRIPTION: The Hikurangi subduction zone (HSZ) of New Zealand is where the Pacific tectonic plate dives beneath the crust of the North Island along its eastern coast. The HSZ displays marked changes in seismicity and plate-slip along its length: to the south the plates appear to be locked and have the capacity to generate great earthquakes whereas to the north the plates are creeping and regularly host episodic slow slip events and the occasional tsunami earthquake. The physical conditions controlling this contrast in plate coupling is not well understood. Recent investigations suggest fluids may play a critical role. To delineate whether fluids act as a primary control on the style of slip expressed along the HSZ, this study will use electromagnetic (EM) methods to image the electrical resistivity structure of the HSZ. Our measurements will provide important constraints on the hydrologic architecture by quantifying the along and across-strike variations in porosity and the fluid budget of the HSZ. This project will support a Ph.D. student and several graduate students, postdocs and early career scientists will be involved in the field work. The PI is an early career scientist who will gain valuable experience leading the field project and mentoring a graduate student. Additionally, a marine EM short-course will be given for students and unfunded collaborators to learn about marine EM theory, applications, data processing, and inverse modeling. This project will use newly proven marine EM imaging technology to constrain the porosity and fluid budget of the crust and mantle along both segments of the HSZ. The survey cruise will deploy a grid of ocean-bottom EM receivers in a 400 km by 150 km area that spans the incoming plate and fore-arc margin. Magnetotelluric (MT) response functions will be estimated from the recorded electric and magnetic field time-series. In addition, controlled-source EM (CSEM) data will be recorded by deep towing a horizontal electric dipole transmitter near the seafloor along three trench-crossing transects located in the southern, central, and northern segments of the HSZ. The MT and CSEM data will be inverted for electrical resistivity and used to quantify the porosity structure across the margin. The HSZ resistivity results will be interpreted in conjunction with existing and planned geophysical and drilling data to test the hypothesis that along-strike variations in seismicity and interplate locking are related to the fluid budget of the incoming oceanic plate, fluid content along the plate interface, and upper plate permeability/porosity structure. The MT data will also image the electrical resistivity of the lithosphere and asthenosphere, providing independent constraints on the origin of the lithosphere-asthenosphere boundary.

OUTCOMES: Click here to see our blog with stories and photos from the successful field campaign: https://emlab.ldeo.columbia.edu/index.php/category/ht-resist/