Scientific Discipline: Geophysics
Research Area: Hikurangi subduction zone, offshore the east coast of the North Island, New Zealand
Research Vessel/Equipment: RV Tangaroa
PI Name and Affiliation: Dr Rebecca Bell, Imperial College, London
Date: 31 March - 11 April 2023
Citation from PI: The EUROFLEETS+ VISIT project will allow us to characterise sediments that will be subducted along the Hikurangi subduction zone and investigate how their character varies from the south (in a region where large earthquakes are expected) to the north (where shallow slow slip events occur). We will be collecting seismic reflection images in a region where no data currently exists to investigate if changes in sediment properties correlate with this change in fault zone seismic behaviour. Part of our project involves searching for sites where we could in the future drill to collect rock samples to help us understand why some subduction zone faults lock-up and then slip in devastating earthquakes and others slip slowly.
Abstract & Main objectives: Subduction plate boundary faults are capable of generating some of the largest earthquakes and tsunami on Earth, such as the magnitude 9.0, 2011 Tōhoku earthquake, Japan. However, in the last two decades a new type of seismic phenomena has been discovered at subduction zones globally: slow slip events (SSEs). These are transient episodes of fault slip that are faster than tectonic plate motion but too slow to incite seismic waves and shaking. The physical mechanisms that lead to SSEs remain poorly understood and their potential to trigger highly destructive earthquakes and tsunami on faults nearby is unknown, making slow slip a new and uncharted aspect of earthquake hazards. Resolving what controls whether a plate boundary fault ruptures in large earthquakes or slips slowly is one of the most important challenges in seismology today. The Hikurangi margin, North Island, New Zealand accommodates oblique subduction of the Pacific Plate beneath the Australian Plate at a rate of 4.5-5.5 cm/yr. Geodetic data reveal that the south Hikurangi plate boundary fault is locked to ~ 20-30 km depth, with deep SSEs occurring down-dip of this zone between 30 and 45 km depth. This deep, strong locking is thought to be a proxy for earthquake potential. The margin exhibits a sharp and profound along-strike transition in interseismic locking at latitude 40oS, with shallow SSEs occurring to the north. The physical properties and processes responsible for controlling the dramatic change in interseismic locking depth and fault slip behaviour at 40oS on the Hikurangi subduction zone are poorly understood.
Our aim is to test whether along-strike changes in the thickness, composition or physical properties of the Eocene-Pliocene pelagic sediments on the incoming Pacific plate are responsible for the sharp, along-strike differences in interseismic coupling and seismic behaviour observed along the Hikurangi margin, and whether similar changes in sediment properties could be responsible for controlling seismic behaviour at other subduction zones. We will do this by collecting seismic reflection data along the Hikurangi margin from south to north.