Getting the best idea of quake magnitude

The area analysed by GNS Science for earthquakes. Image supplied

Seismic activity on the East Coast in 2023 was considered normal, says GNS Science.

“A total of 1455 earthquakes were recorded in the year to date in the Gisborne/East Coast region,” said GNS Science seismic duty officer Sheng-Lin Lin.

“None were detected over magnitude 5.  The largest recorded in 2023 was M4.5.

“Over the past five years, the average number of earthquakes detected each year in this area by GeoNet is 1640. The average magnitude of the largest detected earthquake each year is M5.2.

“So last year, seismic activity in the Gisborne Region was normal.”

In New Zealand, magnitude given to an earthquake is usually a summary magnitude, which is a combination of several different magnitude types that can be estimated most quickly.

“The different magnitude types and the different networks of other agencies provide important perspective on earthquakes in New Zealand,” a statement from GeoNet said.

“If you click on the ‘technical’ tab on an earthquake, (on the GeoNet website) you will see the different magnitudes used.

“Faults that could create earthquakes range in size from less than a metre long to more than 1000 kilometres for the largest earthquakes, and therefore the size of earthquake signals, how far they travel, and how well they are detected vary significantly. Because of this huge range, there is no one way to measure the magnitude of an earthquake that will capture all earthquakes. Seismologists have developed several ways to measure different aspects of an earthquake (different magnitude scales) which allows them to get the best idea of the size of the event and to be able to make direct comparisons with the magnitudes put out by other agencies.

“For large earthquakes ,seismologists use a particular magnitude scale called ‘moment magnitude’ (Mw). This scale measures the low frequency part of the signal and will continue to get larger regardless of how big the earthquake is.

“Moment magnitude is more difficult to calculate as it involves more than simply measuring the size of the signal at a certain frequency and requires a longer portion of the signal. Typically, it will take 20-30 minutes to calculate a reliable moment magnitude which is why it is released later, but it is considered the most reliable magnitude for the largest earthquakes.”