Mapping the along-strike fluid distribution of the Hikurangi Margin, New Zealand using marine electromagnetic methods

New Zealand's Hikurangi margin, the subduction zone located east of the North Island, exhibits intriguing along-strike variation in earthquake slip behavior over a short geographic range. The northern end of the margin is characterized by shallow slow slip events (SSEs) and weak plate coupling while the southern margin exhibits deeper SSEs and stronger plate coupling. These distinctions have, in turn, led to differences in the magnitude and rupture style of the earthquakes experienced in New Zealand. Fluids play a key role in controlling the effective normal stress, and hence locking, at the megathrust plate interface. Therefore, understanding the along-strike distribution of fluids is an essential component in addressing the causes for such observed along-strike variation in earthquake behavior. In early 2019, we collected over 400 km of controlled-source electromagnetic (CSEM) data from 132 ocean-bottom electromagnetometers (OBEMs) and magnetotelluric data from a grid of 164 OBEMs to image the electrical resistivity structure of the Hikurangi margin. Because CSEM data are highly sensitive to the resistivity of the underlying material, and because the resistivity of the shallow lithosphere is a proxy for porosity, this method is particularly suitable for mapping the fluid content along the Hikurangi margin. We present 2-D inversion results for two of the four CSEM profiles collected perpendicular to the Hikurangi margin's northern and southern ends. Integrating our results with seismic surveys of the region, geological and geochemical analyses of drill cores, and a vast onshore MT array will allow us to further constrain the fluid properties of and characterize the plate boundary along the Hikurangi margin.