Revealing the geophysical environment of slow slip using core-log-seismic integration

Subduction zones are located where one of the Earth's tectonic plates slides beneath another - this motion is controlled by the plate boundary fault. These plate boundary faults are capable of generating the largest earthquakes and tsunami on Earth, such as the 2011 Tohuku-oki, Japan and the 2004 Sumatra-Andaman earthquakes, together responsible for ~250,000 fatalities. Although some plate boundary faults fail in catastrophic earthquakes, at some subduction margins the plates creep past each other effortlessly with no stress build-up along the fault, and therefore large earthquakes are not generated. Determining what controls whether a fault creeps or slips in large earthquakes is fundamental to assessing the seismic hazard communities living in the vicinity of plate boundary faults face and to our understanding of the earthquake process itself. In the last 15 years a completely new type of seismic phenomena has been discovered at subduction zones: silent earthquakes or slow slip events (SSEs). These are events that release as much energy as a large earthquake, but do so over several weeks or even months and there is no ground-shaking at all. SSEs may have the potential to trigger highly destructive earthquakes and tsunami, but whether this is possible and why SSEs occur at all are two of the most important questions in earthquake seismology today. We only know SSEs exist because they cause movements of the Earth that can be measured with GPS technology. Slow slip events have now been discovered at almost all subduction zones where there is a good, continuous GPS network, including Japan, Costa Rica, NW America and New Zealand. Importantly, there is recent evidence that SSEs preceded and may have triggered two of the largest earthquakes this decade, the 2011 Tohuki-oki and 2014 Iquique, Chile earthquakes. Therefore, there is an urgent societal need to better understand SSEs and their relationship to destructive earthquakes.
We know little about SSEs because most of them occur at depths of 25-40 km: too deep to drill and to image clearly using seismic data, a remote method that uses high-energy sound waves to probe the Earth's crust. The Hikurangi margin of northern New Zealand is an important exception. Very shallow SSEs occur here at depths of less than 2 km below the sea bed, and they occur regularly every 1-2 years. This SSE zone is the only such zone worldwide within range of modern drilling capabilities and where we can image the fault clearly with seismic techniques - this location provides us with an opportunity to sample and image the fault zone that slowly slips. Drilling provides us with direct information about rock properties and fluid content, however the data is very one-dimensional, and only provides a "pot-shot" glimpse at the subsurface. Seismic data on the other hand images the geometry of the subsurface and allows us to deduce the speed of sound through the rocks over wider areas, however, it does not provide any direct evidence of fluid content or other rock properties. In places where drilling data and seismic data exist together relationships between seismic properties and rock properties can be developed. These relationships can then be applied throughout seismic data volumes to estimate rock properties in areas that have not been drilled. In the case of Hikurangi this would include the deeper part of the subduction plate boundary fault which undergoes slow slip.

This research will have three major classes of non-academic beneficiaries:
(1) Geohazards community and local communities in New Zealand. At an international level, the results from this and the wider Hikurangi projects will improve our understanding of the slow slip process. In turn this will help us understand the fault slip process in general and related hazards (earthquake, tsunami), and the role slow slip might play in potentially leading to earthquakes or tsunami earthquakes. This will significantly impact our ability to assess geohazard potential worldwide with implications for government policy makers and authorities in areas of seismic hazard globally. Local authorities in New Zealand will benefit from improved knowledge of the seismic hazard associated with the Hikurangi subduction margin;.
(2) The general public in the UK and internationally will benefit from being informed about subduction zone and fault slip processes from research-led outreach activities aimed at a broad range of age groups and backgrounds. The international public will benefit from coverage surrounding the results of this study in the media, and the advances made in terms of earthquake and tsunami hazard at subduction zones and potential for the resources industry.
(3) The hydrocarbon and mining industry continue to be keen sponsors of the development of full-waveform inversion (FWI) methods because of the improved imaging of fine-scale structure that this technique allows. Validation of FWI models from subduction zones using drilling data, as is proposed here, will be an important test of the FWI method in regions of deep water and high structural complexity;.

We will interact with these beneficiaries by:
(1) Fully engaging with Bell's colleagues at GNS Science New Zealand, and her links with local authorities in New Zealand. In particular, new rock property information that will be recovered from drilling and integration with geophysical datasets will directly be used to update seismic and tsunami hazard models for New Zealand. Bell will work closely with hazards teams at GNS to make sure they are using the most realistic parameters in their models based on the results of this project.
(2) Bell will update her existing NZ3D FWI website to include information about expedition 375 and provide public education material related to the research. Bell is also intending to give talks at geological societies and festivals around the UK in 2019-2020. Bell will also produce an exhibition for the Imperial College Festival which attracts visitor numbers of ~ 10,000 including government policy makers, industry professionals and the general public of all backgrounds and age groups. In addition, Bell will develop materials suitable for delivery in Imperial College London outreach activities.
(3) Talks will be given at industry-focused conferences including EAGE and AAPG.