Hikurangi subduction zone expedition #375 – blog 6

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 sixth blog from Aliki Weststrate, IODP (International Ocean Discovery Program) Outreach Educator. This is her account of a voyage full of excitement, challenges and science!

Sub-seafloor observatory deployment complete!

There was a lot of celebrating this week as we finished deploying our second (and final) sub-seafloor observatory. This means New Zealand now has two ‘quake-labs’ sitting under the seafloor in the northern Hikurangi subduction zone.

Observatories are notoriously difficult to install due to the deep water depths and tools required, but we are thrilled both were deployed successfully over the 2-month voyage.

Expedition Co-leader Dr Demian Saffer, Pennsylvania State University

The location of the first observatory at site U1518 (Te Matakite – see blog 3) was directly into a thrust fault about 60 km east of Gisborne. Research into slow slip events has also pinpointed this thrust fault site as one where these silent earthquakes might be propagating up to the seafloor, so the scientists chose to position the observatory instruments below, within and above the fault.

A cutaway diagram showing the top section of the first observatory, Te Matakite. The rectangular platform sits on the seafloor. Down below this, the temperature, pressure and chemistry instruments are hanging vertically into the borehole down to 450 mbsf (metres below seafloor).

Slow slip events are an enigmatic phenomenon, only recently discovered by geologists studying Earth’s movement almost 20 years ago. They appear to bridge the gap between typical earthquake behaviour and gradual creeping plate movement.

In the northern Hikurangi region, they occur with remarkable regularity – approximately every 2 years and last over a period of 2–3 weeks at shallow depths (<5–15 km below the seafloor).

The second observatory at site U1519 was closer to shore, about 30 km east of Gisborne. This site was chosen for the observatory because it is very close to the location of a March 1947 earthquake epicentre. This earthquake had low-intensity shaking and was hardly felt on land yet caused a 10 m local tsunami to hit the East Coast, damaging many roads and bridges. The tsunami affected 120 km of coastline from Tokomaru Bay to Mahia Peninsula.

It still perplexes geologists that the shaking wasn’t felt strongly on land yet there was enough uplift of the seabed near the subduction trench offshore to cause a significant tsunami in this area. Installing another observatory into the slow slip source area above the upper plate boundary might shed some light on how and why this occurred.

It’s fascinating to watch Earth scientists from 10 different countries work together to solve the puzzle of slow slip earthquakes here at the Hikurangi subduction zone.

Aliki Weststrate, IODP Outreach Educator

Amazing animals and fun distractions

We’ve been visited by a large pod of dolphins, loads of albatross and seabirds, a fur seal and pilot whales, and a large sunfish drifted by one day, just under the surface.

We are fortunate to have seen a lot of sealife during our 2-month expedition. Watching a pod of pilot whales cruise by this week was a highlight.

Aliki Weststrate, IODP Outreach Educator

We caught some incredible footage of squid this week too – you can watch it here. They are Humboldt squid and are known to attack their own species, but it’s rare to see it on film.

We also celebrate birthdays by having a cake, and on sunny days, we do it outside and have a BBQ near the bow of the ship. Some musical scientists have started a band that makes up songs for people’s birthdays – they’re hilarious.