Slow slips – a detective story

Dr Demian Saffer, Co-chief Scientist on the International Ocean Discovery Program expedition #375, talks to Otumoetai Intermediate School students about slow slip earthquakes. Demian, Dr Laura Wallace and the international crew spent 2 months drilling into slow slip areas to learn more about the underwater geology of the Hikurangi subduction margin.

Note: This video footage was bounced off a satellite during a Skype session on board a working research ship. The beeping in the background is the cryomagnetometer – learn more about this specialised lab machine in the video What do core samples tell us?

If you find the sound quality difficult, please refer to the transcript.

Visit the GeoNet website for an interactive multilayer map that displays the locations of GeoNet’s detection sites.

Transcript

DR DEMIAN SAFFER

It’s kind of an interesting detective story, because one of the characteristics of slow slip earthquakes is that they don’t shake the ground. They go slowly enough that you don’t feel them. They are called slipping, but it takes place over a few weeks or maybe a few months, so the only way to detect that is to have really accurate measurements of that motion.

That’s done primarily with GPS receivers much like what you have in your phone but with a much higher sensitivity so you can measure movements that are maybe a few millimetres. So there is a network which is on land in New Zealand, and there are also a few much more complicated bursts of those that can take place off shore, temporarily. And it is those GPS instruments that actually show us that the land is moving, sort of skidding along, roughly every 2 years for about 10–20 centimetres. Then it stops, and then a few years later, it does it again. That field is called geodesy. It’s using the GPS measurements to basically see the fault moving and stopping at slow speeds, and it is really the only way we can do it.

Regular earthquakes, you mainly measure magnitude by the amount of shaking. With slow slip events, they don’t shake the ground and so we have to use a different method. The way that is calculated is just how much the fault slipped, how big the pact was, how big the area was that was moving and how much it moved, and those together find how much energy was built up and then relieved in the earthquake. Then again it is taking place really slowly so you don’t feel it.

One of the primary people who really discovered these in New Zealand is my colleague and other co-chief scientist, Laura Wallace.

Acknowledgements Dr Demian Saffer, Pennsylvania State University Aliki Weststrate International Ocean Discovery Program (IODP) Australia and New Zealand International Ocean Discovery Program Consortium (ANZIC) GNS Science Otumoetai Intermediate School

Diagram of Hikurangi Trench, GNS Science

Screengrabs of GeoNet seismic monitoring network, GNS Science, CC BY 3.0 NZ

Photo of Dr Laura Wallace and Dr Demian Saffer, Tim Fulton, IODP and

JOIDES Resolution Science Operator

All other footage from ship to shore video conference from JOIDES Resolution expedition #375 courtesy of Otumoetai Intermediate School