M5.3 Eketāhuna earthquake and a Ruatōria ghost
For those shaken awake by Tuesday nights M5.3 Eketāhuna earthquake some of you may have spotted a M5.8 earthquake near Ruatōria at the same time, on our app and website, which then disappeared. Read on as we explain what happened and what ghost earthquakes are.
The Eketāhuna earthquake
The M5.3 earthquake occurred at 11:26pm, 10km East of Eketāhuna, with strong shaking intensity and was felt over 23,000 people.
Felt reports and Shaking Layers from the M5.3 Eathquake
This earthquake occurred in a seismically active area of Aotearoa New Zealand that extends from Wellington to the Gisborne region. It is a geologically complex area where the Pacific tectonic plate is being subducted beneath the Australian plate at a rate of approximately 42mm per year.
At 35km deep this earthquake was in the subducting slab and occurred in a region dominated by normal faulting. In the first day following the earthquake we located three aftershocks.
Those in the area will probably remember the M6.2 Eketahuna earthquake that occurred on Jan 20 in 2014, you can read more on that from our 2024 story: 10-year anniversary of the M6.2 Eketāhuna earthquake
The making of the Ruatōria ‘ghost’ quake
On Tuesday night a few minutes before the Eketāhuna earthquake, at 11:22pm, our instruments detected another real earthquake 960 km north-east of Whangarei and the combination of these earthquakes caused our automated system to post a ‘ghost’ M5.8 earthquake in Ruatōria to our website and app.
Our automatic earthquake location system is set up to best locate earthquakes within New Zealand. Ghost quakes can occur when the location software interprets the waves arriving from a distant earthquake and reads them as multiple local earthquakes.
As our seismic equipment is very sensitive to vibrations and can detect seismic waves from earthquakes that happen thousands of kilometers away. For very large earthquakes the waves can even be tracked circling all the way around the world and back again!
The confusion in the software system is even more likely when multiple earthquakes occur within minutes of each other and waves from two or more earthquakes are incorrectly identified as belonging to a single earthquake. This is what happened on Tuesday night. This is one reason why we still use analysts to review and locate earthquakes as they occur rather than relying on a fully automated system.
When signals are isolated in time from other earthquakes, our automated location software performs reasonably well even for earthquakes far offshore. However, when signals from two earthquakes arrive close together in time, the first arrival of one can easily be mistaken for the second arrival of the other. This leads to an incorrect estimate of the primary measure of distance between the earthquake and the station recording the data, resulting an incorrect location.
The figure below shows the arrival of the earthquakes at our seismic station near Matawai (MWZ). There is an earlier earthquake to the west of New Zealand in the Tasman Sea, and then very closely in time earthquake waves from the 11:22pm Kermadec earthquake and the 11:26pm Eketāhuna earthquake. These arrivals are likely part of what created a ’ghost quake’ that the system automatically assigned to be near Ruatōria.
Plot showing 24 hours of seismic data from a seismometer near Matawai showing separate earthquakes arriving at the station close in time.
We work hard to quickly fix these automatic events, but they can cause confusion for those who happen to see them, and Tuesdays Eketāhuna earthquake meant lots of people were looking at our app and website and saw this ‘ghost’ in addition to the real earthquakes at Eketāhuna and north of New Zealand in the Kermadecs. When our geohazard analysts review these simultaneous earthquakes or distant detections, they remove possible duplicates created by our system and revise the origin and magnitude of the real earthquake using the data. Almost always this results in the earthquake source moving hundreds of kilometres away into the Kermadec Arc to the north-east of New Zealand.
Getting you the information as soon as possible
Ghost quakes are a side effect of us wanting to get information out as quickly as possible. Instead of waiting for an analyst to check each earthquake manually, the automatic locations are published to our website and app immediately while manual checks are happening in the background. Although occasionally the automatic locations are these misinterpreted ‘ghost quakes’, the majority of the time automatic locations are a good first indication of the earthquake location and publishing the locations before they are reviewed ensures we get information about real earthquakes out as rapidly as possible.
Earthquakes can occur anywhere in New Zealand at any time. In the event of a large earthquake: Drop, Cover and Hold.
Drop, Cover and Hold
Remember Long or Strong, Get Gone : If you are near the coast, or a lake, and feel a strong earthquake that makes it hard to stand up OR a weak rolling earthquake that lasts a minute or more move immediately to the nearest high ground or as far inland as you can, out of tsunami evacuation zones.
Know what to do?
The National Emergency Management Agency (NEMA) has a great website with information on what to do before, during and after an earthquake.
Prepare your home. Protect your whānau.
There’s a lot we can do to make our homes safer and stronger for natural hazards. Natural Hazards Commission Toka Tū Ake's website has key steps to get you started.