Another silent earthquake or slow-slip event is just getting underway off the coast of Gisborne. These silent earthquakes are undetectable by both humans and GeoNet's seismographs but they can move faults the equivalent of magnitude 5+ earthquakes. A normal earthquake is over in less than a minute, but these East Coast silent earthquakes take anywhere from a week to a month to unfold.This most recent one started around a week ago, and they happen every year or two. You can keep an eye on this one unfolding with this auto-updating graph, or by checking our 'Slow-slip watch' webpage.

Auto-updating graph of two GPS stations - Makorori and Gisborne. Note: data takes 2 days to process.

So if we can't record them on a seismograph, how do we know these quakes are happening? We're able to record these silent quakes using GeoNet's network of GPS stations (the plot above is from two of those stations). Like lots of scientific discoveries, these silent quakes were stumbled upon while investigating something else entirely. They were first discovered in North America a few decades ago, and only discovered in New Zealand in the early 2000s when GeoNet (in partnership with LINZ) installed GPS stations around the North Island for mapping and surveying.

How do these silent earthquakes relate to normal earthquakes? This is a pretty new area of study so we're always learning more. In the past we've sometimes seen swarms of small (magnitude 2 to 4) earthquakes associated with silent earthquakes that happen around the periphery. It's likely that a magnitude 4.0 quake offshore of Gisborne last week is related to the current silent earthquake.

There is worldwide interest in studying our subduction margin (where one plate dives beneath another). In terms of logistics, it's one of the easiest to study, as it's close to shore. Another factor that draws international researchers, is GeoNet's policy of making all data freely available to everyone. Japanese researchers from Kyoto and Tohoku Universities are currently out on a research vessel retrieving instruments that have been recording for a year, as well as installing more instrumentation to look at how the seafloor moves and deforms over time. Want to know more? We've written lots of stories explaining what we know about silent earthquakes. The story we wrote during the last Gisborne silent quake in 2014 has some great detail.

How common are these silent earthquakes? Silent earthquakes happen every few years in this part of the country. In New Zealand, they also happen in Hawke's Bay, Manawatu, and Kapiti. The Manawatu and Kapiti events are generally bigger and longer-lived, but happen less frequently (every 5 years). Silent quakes happens in other parts of the world with subduction margins, e.g. Japan, Costa Rica, Mexico, and Cascadia.

If more happens around this current silent quake, we'll let you know!