Hikurangi subduction zone expedition #375 – blog 3

In March 2018, JOIDES Resolution, a large scientific research vessel, headed out to sea to research the Hikurangi subduction zone on expedition #375. This article is the third blog from Aliki Weststrate, IODP (International Ocean Discovery Program) Outreach Educator. This is her account of a voyage full of excitement, challenges and science!

Week 3 went quickly because there was a lot of action installing the first our two sub-seafloor observatories. We also celebrated Easter, with some pretty funny Easter bunnies.

The first observatory was named by Gisborne Boys’ High School student Matthew Proffit – Te Matakite (to see into the future).

Putting in an observatory under the seafloor is like sending a satellite to space – it is difficult to do and involves a huge range of people: engineers, scientists, drillers and observatory specialists.

Why are we putting observatories in under the seafloor?

There are three reasons:

• It is really rare to have shallow earthquakes at subduction zones, but 70 km off Gisborne the subduction zone is shallow enough for us to drill a hole into and insert an observatory – like a probe.
• New Zealand doesn’t yet have any observatories – other subduction zones around the world do (like Japan, Costa Rica, the Marianas), and they are helping geologists understand why and how earthquakes happen where tectonic plates meet.
• In blog 2 I mentioned the northern Hikurangi subduction zone is an area where slow-slip earthquakes are happening. These type of slow earthquakes (which we don’t feel and that happen over weeks, not seconds) were only discovered 15 years ago, so we need to learn more about them and their relationship to big earthquakes that could cause tsunamis.

The stages to installing New Zealand’s first observatory under the seafloor

During week 2, we put in the free-fall funnel, and this week, the next stages went in successfully - after some nail-biting moments.

The process is especially complex because we are aiming to install two different monitoring packages – one inside the other, sort of like a Russian nesting doll.

The first monitoring package consists of pressure sensors attached to the first stage of the observatory. These will measure pressure changes below, in and above the fault over years.

The second monitoring package consists of temperature sensors and coils that will collect fluids. It will be suspended by a rope inside the second stage of the observatory. The coils measure chemical changes and are really advanced. Not all observatories measure chemistry as it is difficult to do. The rope is attached to a plug that seals the top of the smaller inner casing and leaves the observatory there for up to 5 years to record slow-slip events over time.

To monitor temperature and chemistry changes in the borehole, the borehole must be isolated. We do this by closing the top of the hole with a wellhead and the bottom of the hole with a bridge plug. We further isolate the fault zone with packers. These are long elements made of rubber material that will expand once submerged in water and will seal off the fault interval inside the observatory casing from the intervals above and below it. Once this is done, we lower in narrower casing and a second smaller wellhead.

It sounds easy when it is described and drawn like this, but remember that we are in 2,640 metres of water, with the steel pipe being moved about like spaghetti in the water under the ship! It’s an amazing engineering feat and very challenging to install.

How do we get all the data?

In a few years, we will return to site U1518 with another vessel to retrieve the data using a remotely operated vehicle (ROV) – a robotic submarine. This will connect to the wellhead with a cable and download the pressure data and send it back up to a computer on the ship.

The next step is harder. To access the temperature and chemistry data and samples, we have to pull up the temperature logger and OsmoSampler string using the ship’s winch. If portions of the string are stuck and cannot come up, there are three weak links in the string designed to break at different forces so that a portion of the string can stay behind and the rest of it can be retrieved.

What can go wrong?

A lot! I had little idea of how complex an operation this was before I came on board nor how many things need to align and come together to install Te Matakite. Thankfully, our first observatory is now safely in the ground, 450 m deep in the Hikurangi subduction zone - and with 2,640 metres of water on top of that!

Common mishaps include the borehole collapsing and trapping the pipes, tools not working correctly or getting jammed and drillers not being able to get the pipe back into the re-entry cone or damaging things in the process of trying. We also needed mild weather with a calm sea so there was not too much heave (the up and down movement a boat experiences in a swell).

The social and scientific impact

We hope that Te Matakite and a second observatory will pave the way to long-term measuring equipment being installed in New Zealand’s Hikurangi subduction zone, which could act as an earthquake warning system in the future.

The puzzle we are putting together on JOIDES Resolution will help subduction zone scientists all around the world understand earthquakes and the slow-slip phenomenon better.

Aliki Weststrate

For people living near subduction zones, it will be used to improve risk models of earthquakes and tsunamis. Better risk models will lead to improved hazard preparedness and fewer lives lost.