Structures and deformation mechanisms in a slowly slipping subduction thrust, Hikurangi Margin, New Zealand (IODP Expedition 375)

Slow slip events are a recently discovered phenomenon, where slip occurs at a rate faster than plate boundary deformation, but too slow to generate seismic waves. The discovery of these slow slip events has challenged the long-held paradigm that faults either accumulate displacement steadily, at rates of centimetres per year, or fail episodically in potentially damaging earthquakes. Instead, there is now evidence that faults can slip at rates that very across a huge spectrum, from slower than fingernail growth to the meter per second velocity that occurs in damaging earthquakes. We do, however, not understand the physical controls on this range in fault slip behaviour.

The Hikurangi Margin, New Zealand, presents a unique opportunity in drilling into a fault that is known to accommodate slow slip events. Whereas most slow slip has been detected at depths in excess of ten kilometres, out of range of ocean drilling, the Hikurangi Margin experiences slow slip events that propagate to depths less than 2 km, and maybe even all the way to the sea floor. Observatories to be installed during expedition 375 will better define the geometry, but critically to this proposal, expedition 375 will also sample the materials that host the plate boundary fault, and therefore also the slow slip events.

With samples from expedition 375, I will investigate the rocks that host slow slip events in the Hikurangi Margin. Using state-of-the-art scanning electron microscopy at Cardiff University, the composition and microstructure of the materials entering the subduction zone can be studied in great detail. The planned boreholes intersect the ocean floor and the overlying oceanic and terrestrial sediments, allowing us to know what rock type(s) slip is(are) occurring in. A borehole will also intersect the shallowest inferred active fault at the front of the accretionary prism, allowing insights into how the sediments deform when faulted.

Specifically in this proposal, I propose to characterise the structures that have developed in the incoming sediments and oceanic crust as these rocks enter the subduction zone. This characterisation will show how deformation is distributed at the very shallowest level of the plate boundary, and allow inferences of where in the incoming sequence the plate boundary fault localises. Importantly, seeing what structures have developed at the microscale also allows inference of what deformation mechanisms are active in each sampled rock unit. This inference lets us know what rocks are likely to fracture, and what rocks may deform by other mechanisms. This information is important in understanding how some materials may host earthquakes, and others slow slip events.

The key aim of IODP expedition 375 is to investigate the materials and in situ conditions that allow subduction zone slow slip events to occur at the Hikurangi margin, New Zealand. The main potential beneficiaries for such research are:

1. Communities and policy makers in regions of seismic hazard. This is because an improved understanding of faults that host slow earthquakes can help bridge the knowledge gap in what controls the speed at which faults slip. Making sure the research can reach seismic and tsunami hazard teams will be a joint effort of the science team on Expedition 375, working along side onshore collaborators. Specifically for this moratorium proposal, work on recognising the type of sediments and fault rocks that may host slow earthquakes will be communicated to existing collaborators in New Zealand and Japan, and results presented at major international conferences such as the American Geophysical Union Fall Meeting.

2. More specifically, local New Zealand communities will gain improved understanding of their offshore geology, and particularly an improved understanding of seismic hazard related to the Hikurangi subduction zone. Reaching these communities will occur through existing links with the governmental Earth Science agency GNS Science and New Zealand participants on Expedition 375.

3. Education and outreach activities have long been central to IODP expeditions, and through participating in the expedition, and undertaking post-cruise research, I will contribute to creation of education and outreach materials, and presentation of such materials in the UK. Two outreach specialists sail on the expedition to assist with creation of outreach materials. There is also tradition and history for IODP expeditions producing outreach material onboard. The web portal in particular encourages broader participation of the general public in the science occurring on board the ship during expedition 375.