Magnitudes are on a logarithmic scale, which means that each whole number increase represents a tenfold increase in measured amplitude. Photo: 123RF
Wednesday's magnitude 8.8 earthquake off the coast of far eastern Russia is one of the largest in years, raising fears of a tsunami around the Pacific.
It was the strongest quake since 2011, when a magnitude 9 megaquake struck northeast Japan, killing nearly 20,000 people. If it holds, it might be one of the 10 largest earthquakes ever recorded.
Magnitudes are the size of the earthquake, usually based on the amplitude of seismic waves recorded on seismometres.
Magnitudes are on a logarithmic scale, which means that each whole number increase represents a tenfold increase in measured amplitude.
A magnitude can be adjusted once more seismic readings come in - the Russia earthquake was originally listed as magnitude 8, but upgraded to 8.8.
Other factors can influence how severe an earthquake is, including its location and depth.
The Christchurch earthquake that killed 185 people in February 2011 was magnitude 6.2, but struck a densely populated centre.
The 2004 Indian Ocean earthquake and tsunami that left more than 225,000 people dead was triggered by a quake with a magnitude of 9.2 to 9.3.
- Read more about earthquake magnitudes here from GeoNet.
Damage from the Myanmar quake earlier in 2025. Photo: AFP/ Xinhua - Myo Kyaw Soe
How world's biggest earthquake affected NZ
The largest earthquake reliably recorded was a magnitude 9.5 off the coast of Chile in 1960.
The subsequent tsunami traveled across the world and caused coastal evacuations around New Zealand.
The effects were observed at more than 120 locations, GeoNet reported.
"The most affected locations occurred along the whole eastern seaboard from Cape Reinga to west of Bluff and to Stewart Island, but the tsunami was also observed at locations on the west coast of the North Island including Ahipara in Northland, and notably as far south as Whanganui and Paremata, but not at New Plymouth, Foxton, or Himatangi Beach."
It triggered the setup of the formal tsunami warning system in 1965, NIWA researcher Rob Bell told the East Coast Lab.
"Our tsunami warning system was virtually non-existent. Following the event, negotiations to connect New Zealand to the Pacific Tsunami Warning Centre and a formalised warning system were concluded in 1965," Bell said.
"It struck New Zealand blind," distinguished professor of disaster management David Johnston told RNZ last year.
"It hit most of the east coast of the North and South Islands, inundating properties in Napier, Gisborne, and Lyttelton [and] caused considerable damage.
"No loss of life, but a higher tide would most likely have led to multiple fatalities."
The Canterbury Earthquake National Memorial. Photo: RNZ / SIMON ROGERS
By the way, there has never been a magnitude 10 earthquake ever recorded, and there probably never will be.
"No fault long enough to generate a magnitude 10 earthquake is known to exist, and if it did, it would extend around most of the planet," the US Geological Survey says.
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