Collaborative Research: A community 3D seismic investigation of fault property controls on slow-slip along the Hikurangi megathrust

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Subduction fault zones, where one of Earth's tectonic plates is forced beneath another plate, produce the most powerful and destructive earthquakes and tsunamis on the planet. What controls this fault slip behavior is virtually unknown and yet it is critical for understanding and assessing global seismic hazards. While these great subduction zone earthquakes release seismic energy in a matter of seconds to minutes, recent discoveries reveal another style of fault movement called slow slip earthquakes (SSEs) that take days, weeks or months to occur. While not as damaging as major earthquakes, these slow slip earthquakes are nonetheless thought to be a critical process in how strain builds up and is subsequently released on subduction zone fault systems. Slow slip earthquakes may also act as indicators of where, and possibly when, large damaging earthquakes might occur. The subduction zone along the east coast of the New Zealand North Island regularly (~ every 4 years) produces unusually shallow (4-5 km below the sea surface) slow slip earthquakes compared to the typical 20-50 km depths found in other areas such as the Cascadia and Alaska subduction zones on the west coast of the U.S. This project will conduct a 3D seismic survey that is designed to image the shallow slow slip region of the New Zealand subduction zone, map the fault systems, determine what conditions are associated with slow slip behavior and how they differ from conditions associated with subduction zones that generate great earthquakes. The project includes substantial international collaboration with New Zealand, Japanese and United Kingdom researchers on the seismic imaging as well as direct links with an international ocean drilling program designed to drill into the zone of slow slip. The project involves an early career scientist and several graduate students who will work on various aspects of the research for their thesis work. Additional students will be involved in the field data collection from all participating countries. Public outreach in the form of media coverage, news stories, public lectures and ship tours will be coordinated with New Zealand organizations and collaborators.Slip on plate boundary faults associated with recorded earthquakes typically fails to account for all of the expected plate motion predicted by geodetic methods or global models. Exciting advances in the last decade have recognized and documented transient slow slip earthquakes as another mode to accommodate plate convergence at subduction margins. This project addresses the mechanics of subduction thrusts and the conditions that lead to the wide variety of slip behaviors by carrying out the first-ever 3D seismic dataset acquired over a subduction thrust dominated by aseismic creep. The project will use the research vessel R/V Langseth to acquire an open-access 3D seismic reflection data set in a 15 x 60 km area offshore New Zealand's Hikurangi trench and forearc. These data will provide high-resolution images and seismic attributes of a transient-slip plate boundary megathrust identified by the research community as an important point of comparison to predominantly locked subduction thrusts such as Nankai, Tohoku, Cascadia, and Costa Rica. Seismic attributes linked to high fluid pressures are a promising indicator of conditions favorable for slow slip earthquakes and the Northern Hikurangi margin has well-documented slow slip earthquakes at 4-5 km depth - shallow enough for detailed seismic imaging and seismic attribute measurements. The 3D seismic images and attributes will allow for an accurate documentation of the structural, stratigraphic, and hydrogeologic conditions that lead to generation of slow slip earthquakes along a subduction megathrust. International collaborative experiments will record Langseth shots during 3D acquisition and develop the first ever high-resolution 3D velocity models across a subduction zone using 3D full-waveform inversion, which will overlap and extend beyond the 3D volume.