A slow-slip event, also known as a “silent earthquake”, has been unfolding in the Hawke’s Bay region since early May, causing around 2cm of eastward motion over the past two weeks.
That’s the equivalent of around six months of plate motion condensed into 14 days.
Unlike usual earthquakes, these slow-slip events take place over weeks or months rather than seconds.
GNS’ science operations technical lead Elisabetta D’Anastasio said the slow release of energy meant it did not cause shaking noticeable by people or seismographs.
“Instead, it is our continuously operating Global Navigation Satellite System (GNSS) stations (commonly referred to as GPS) that track millimetre-level movement of the land on a daily basis, that allow us to see events like this unfold.”
This type of activity was not unusual, D’Anastasio said.
“When slow-slip events occur, they relieve a lot of pent-up stress on the subduction plate boundary. At the same time, they can increase stress on other nearby portions of the plate boundary, so they are occasionally accompanied by an increase in smaller earthquakes that may be felt by the public.”
As a result, more than 100 earthquakes had been located in the central Hawke’s Bay region since the beginning of May. The largest event has been a 3.1 magnitude earthquake on May 17.
Slow-slip events were common in the Gisborne and Hawke’s Bay area, D’Anastasio said.
“The most recent was reported in December last year offshore the Gisborne region, between Mahia Peninsula and Tolaga Bay – that lasted about a month.”
She said the current event was picked up between the GNSS stations at Cape Kidnappers and the Mahia Peninsula.
“Based on our observations of past Hawke’s Bay slow-slip events, this one is likely to continue for the next few weeks.”
While the slow-slip events did not pose a risk, D’Anastasio said they were a “major part” of growing understanding on how tectonic plates move in a subduction zone.
“If we can better understand slow-slip events, we can better understand the earthquake potential of subduction zones.”
It also served as a “good reminder” of New Zealand’s position straddling a “very active tectonic plate boundary”, she said.
“The best thing you can do now is refresh yourself on earthquake preparedness information. Our team will continue to keep an eye on this slow slip activity, and we will keep you updated as new data comes in.”