Published: Tue Jun 24 2014 12:00 PM
Multiple “ghost quakes” have appeared on our seismic sensors after the 6.9 Kermadec Islands earthquake (revised from 7.2) shook Raoul Island earlier this morning (24 June 2014).
What is a “ghost quake”?
“Ghost” quakes appear on our network typically after a large regional source earthquake. We have very sensitive seismic equipment that picks up the various waves that earthquakes create and we can pick up these waves even if it is very far away. For example, we also picked up seismic waves from the Alaska quake this morning. Our equipment gets confused by these waves and interprets these as being a smaller, locally-sourced earthquakes close by.
Why do we get “Ghost Quakes”?
These quakes are an unfortunate side effect of getting information out to the public as quickly as possible, instead of waiting up to a quarter of an hour for a person to locate and ensure these are authentic earthquakes. This started when we introduced “GeoNet Rapid”; the up side is that the system is highly efficient and quick with earthquake reporting times of just minutes after the quakes. It is sharply arriving S-waves - that our automated system confuses for P-waves - that are causing the incorrectly reported earthquakes. Our seismologists are easily able to see the confusion and gradually mop up the false earthquakes on the website.
How can we stop “Ghost Quakes”?
We are working on it but it's tricky. If we make the system too picky on the quakes it reports, we might not get rapid information about real earthquakes that occur. If we let it report on whatever information it picks up, we get “ghost quakes”. At GeoNet, we have erred on the side of speedy reporting of everything because we know how important it is to get information as quickly as possible to everyone. Having said that, we are working on finding out what we can do to better identify them and so exorcise “ghost quakes” from our system. Until then, please be patient with us, and keep up the sense of humour, New Zealand!