Understanding slow slip earthquake mechanisms and seismic hazard using local earthquakes: Hikurangi margin, New Zealand

Subduction zones produce some of the largest and most destructive earthquakes and tsunami on Earth. However, in the last two decades a completely new slip mechanism has been discovered, in which slip occurs faster than the plate motion rate but too slowly to produce seismic waves. These events are called slow slip events (SSEs) and given their recent discovery we do not yet know whether SSEs inhibit or increase the likelihood of a future large earthquake with damaging ground shaking. Laboratory and numerical modelling experiments suggest that fluid build-up is needed to facilitate this slow slip. What is currently missing are direct measurements of physical properties across the plate boundary fault zone and the rocks lying above slowly slipping faults to establish if fluid build-up (or high pore-fluid pressure) really does exist in these areas.

The Hikurangi subduction zone, along the east coast of the North Island of New Zealand hosts some of the most well-characterised and shallowest SSEs in the world. Lying above the zone of SSEs is the town of Gisborne, which has experienced violent mud volcano eruptions, gas seepage and land surface instability. These geological features point to fluids and high fluid pressure in the subsurface; however, limited high-resolution geophysical studies exist to confirm whether this is the case. Fluid build-up at shallow depths (<3 km deep) causing these instabilities and fluid escapes may also be linked to deeper fluids related to the slow slip events. In 2017-2018 an array of 50 broad-band seismometers and 145 short-period instruments were deployed across the north Hikurangi margin. These arrays recorded the many local earthquakes that occur in this seismically active region and have captured information about subsurface conditions two months before a major mud volcano eruptuion in Dec 2018.

We are looking for a student with a background in geophysics and interest in tectonic processes to analyse this large dataset to learn about physical properties and anisotropy along the Hikurangi margin to better understand the structure of the region and the implications for mud volcanoes and slow slip. The student will i) develop a local earthquake catalogue, ii) conduct tomography to develop a velocity model from the local earthquake data, iii) use earthquake splitting techniques to learn about anisotropy and iv) interpret these findings in light of the regional geology and tectonics. The research deliverables, namely recovery of the physical properties of the fault zone and overriding plate, can be used to directly update seismic risk assessments for New Zealand and other locations that have slow slip events. The student will spend time in New Zealand working with project partners at GNS and conduct field observations in the Gisborne area. While in New Zealand the student will be encouraged to present their results in town-hall meetings to audiences including local councils, iwi, civil defence and general public.