GeoNet has upgraded the magnitude of the earthquake at Lake Taupō on November 30 based on updated data around the ground shaking, landslides, volcanic activity and tsunami that occurred.
The earthquake, previously reported as a 5.6 magnitude, has been raised to 5.7 with over 680 aftershocks being recorded, with the most recent being on Monday.
GeoNet says this change reflects the greater accuracy of their measurements since the event, with this number of aftershocks being more in line with a 5.7 quake.
"The magnitude and rate of aftershocks has started to decline but is expected to continue for several weeks," GeoNet says.
The probability of another earthquake of magnitude five or above in the next 30 days is estimated to be very unlikely at 11%.
GeoNet says a small tsunami was generated by the quake in Lake Taupō which pushed debris almost a metre above lake level at parts, causing a major washout of the foreshore and beach at Wharewaka point and leaving pumice 40 metres inland.
Over 30 landslides were triggered by the magnitude 5.7 quake, which GeoNet says "is not unexpected, given the severity of the shaking, and the timing of the earthquake, which followed several weeks of particularly wet weather".
Most of these were small slips of steep slopes near roads but larger rockfalls were identified closer to the epicentre by Hatepe.
On the White Cliffs along the eastern lake shore, "a several-hundred-metre-long (though relatively shallow) section of the cliffs collapsed into the lake, generating a large white plume of sediment that could still be seen stretching north along the coastline several days after," GeoNet says.
They are investigating the possibility than an underwater landslide occurred at Wharewaka Point, causing 170 metres of the shoreline to subside.
"Underwater landslides are known to be some of the largest landslides on earth and can trigger tsunamis, however, there is currently no evidence to suggest the Wharewaka Point landslide generated the larger lake-wide tsunami," GeoNet says.
Although minor volcanic activity has been happening at Taupō Volcano since May 2022, GeoNet says it's within the anticipated range and is consistent with "minor volcanic unrest", not warranting a raise to the volcanic alert level.
Before November's quake, they say "the unrest had been characterised by hundreds of small, typically non-felt, earthquakes and a few larger events (M3.5 to M4.2)...none of these earthquakes are volcanic types."
Taupō has recorded 18 periods of volcanic unrest in the last 150 years and none of them have led to an eruption.
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