Google's earthquake alert system warned Hawke's Bay residents' that a 6.0 magnitude quake had hit off the coast.
Google's earthquake alert system warned Hawke's Bay residents' that a 6.0 magnitude quake had hit off the coast.
It wasn't just the 5.1 magnitude earthquake which woke many residents across Hawke's Bay on Monday morning.
Many bleary-eyed Android users were also left in a state of confusion after receiving a Google mobile phone alert which described it as an estimated 6.0 magnitude quake off the coast.
The inaccuratealert, generated by Google detecting widespread shaking of cellphones across the region, had some wondering whether they needed to head to higher ground.
Government-funded alerts system GeoNet reported the 5.1 magnitude quake was centred about 10km south-east of Tokoroa in the Waikato region and struck at a depth of 160km shortly after 1am on Monday morning.
It was widely felt across New Zealand, particularly by those in the North Island, with almost 2800 people recording their responses on the GeoNet website.
A social media post by GeoNet explained deep earthquakes, such as this one, were typically widely felt, but not usually close to the centre.
"This is due to the subducting tectonic plate under the North Island - most of the quake's energy travels up and along the plate to the surface - closer to the east coast of the country," it read.
Google detected Hawke's Bay residents' phones shaking, and then alerted them to tell them a 6.0 magnitude quake had hit off the coast. Photo / Supplied
Dozens of residents across Hawke's Bay took to social media after being confused by the alert issued by Google, which described the quake as a 6.0 magnitude quake and put the epicentre off the east coast of New Zealand.
Hastings-based Jill Fallowfield said she was woken by her cat meowing in her face.
"I'm guessing she could feel it coming so I got up to see what was up with her and felt the jolt.
"A few minutes later I got the Android emergency message that said it was out at sea so looked at GeoNet, who said Tokoroa."
She said it was confusing and left her wondering whether there had been two separate quakes.
Others, commenting on a post by Hawke's Bay Today, said they had been surprised by the alert. Many said they did not know about the system.
One woman said she heard the quake and felt her house creak but did not receive the alert and was "gutted" not to get it.
Others said they were relieved not to have gotten it or been woken by the alert as they would have "freaked out".
WHAT IS GOOGLE DOING?
The alerts were sent out to Android phone users as part of the Android Earthquake Alerts System, a trial launched in NZ and in Greece earlier this year.
It uses a physics-based approach to determine the earthquake magnitude and location.
Accelerometers built into most Android smartphones are used to detect seismic waves which indicate a potential earthquake with this signal sent to Google's earthquake detection server, along with a coarse location of where the shaking occurred.
The server then takes this information from many phones to figure out if an earthquake is happening, where it is and what its magnitude is.
Alerts are only issued for earthquakes estimate be a magnitude 4.5 or larger and are different from the National Emergency Management Agency system used to alert mobile phone users about Covid-19 lockdowns, as well as tsunamis and earthquakes.
Google has been approached for comment about the use of the alert in regards to the magnitude 5.1 quake, as well as its lack of accuracy.
GeoNet reported it as a 5.1 magnitude earthquake centred near Tokoroa, while the Google Alert described it as a 6.0 magnitude earthquake off the east coast of New Zealand. Photo / GeoNet
GEONET WASN'T IMMEDIATELY RIGHT EITHER
John Ristau, a GNS Science seismologist, explained earthquakes measured by GeoNet go through a series of automatic and preliminary solutions.
"In this case the first automatic solution was magnitude 3.4 which was reviewed, based on additional data, to magnitude 5.1 as the best solution."
"Having an initial magnitude lower than the final magnitude is not uncommon for deep earthquakes.
"Nearby stations typically underestimate the magnitude of deep earthquakes, sometimes by quite a bit, and we have to wait for data from further away stations to get a proper magnitude."
On the Google alert confusion, Ristau said he welcomed any innovation which helped build the country's resilience to earthquakes, which complemented the crucial work done by GNS Science and GeoNet.
The National Emergency Management Agency is the official agency for providing advisories and warnings for New Zealand.
A spokesman for the agency said they had not been involved in the development of the Google earthquake early warning system, but it was intended to provide people with a few seconds heads up that earthquake shaking may be imminent.
This helped people "mentally prepare" and take immediate life-safety actions such as drop, cover and hold, before seeking further information from official channels.
He said GeoNet continued to be "the definitive source" of earthquake information for New Zealand.
"We encourage everyone to download their app as it is an easy and fast way to check the details of all earthquakes located in and around New Zealand."
National advisories and warnings, when there is a serious threat, are published on its website civildefence.govt.nz, and Twitter @nzcivildefence.
