Magnitude 7.3 earthquake actually two quakes which caused separate faults to break, GNS finds
Friday, 12 March 2021
The magnitude 7.3 East Cape earthquake was actually two earthquakes, a short distance and a few seconds apart, GNS Science says.
“What we know so far is that the earthquake broke two large faults in different locations and at different depths,” GNS seismic modeller Caroline Holden said.
Recordings from land-based GPS instruments show the 7.3 magnitude quake on March 5 pushed large parts of East Cape and Hawke’s Bay about 1cm to the west and southwest, which was not unexpected for an earthquake of that size.
Seismologists around the world were deciphering the various signals the earthquake produced, GNS said.
**READ MORE:
* 'Moderate' 6.1m earthquake reported off east coast
* Tsunami-triggering earthquakes could cause aftershocks for weeks, expert says
* The Kermadecs - a sub-tropical paradise with earthquakes, volcanoes and tsunami
**
Tsunami wave amplitudes, ground shaking recordings, and GPS measurements of ground displacement were helping to build a complete picture of the event.
The East Cape event at 2.27am was followed by two earthquakes in the Kermadec Islands - a 7.4 magnitude event at 6.41am and an 8.1 magnitude event at 8.28am.
It was extremely rare for three earthquakes in quick succession to generate three tsunamis that then combined in complicated ways to impact on one country, GNS said.
It was so rare that most tsunami scientists could not recall any similar examples.
In the past week, tsunami specialists at GNS have been collecting and analysing data collected from deep ocean tsunami sensors, tide gauges, and eyewitness accounts.
“They are starting to form a clearer picture of the three tsunamis and how they interacted to threaten our coasts. In particular, the data is expected to shed light on aspects such as the length of time a tsunami threat can last and the level of hazard it represents to different stretches of coast,” GNS said.
GNS is keen to hear from anyone with information about any unusual sea behaviour on March 5 or 6. It has put up a questionnaire online where people can provide the information, and is particularly keen to get photos or videos of the sea at the time.
Monitoring volcanoes
A close watch was being kept on active volcanoes in case there was any change following the earthquakes, but so far there was nothing significant to report, GNS said.
“We haven’t seen changes at Whakaari/White Island, but at Raoul we have seen a small rise in the level of Green Lake of about 150mm,” volcanologist Brad Scott said.
'We have observed a landslide into the pool at Marker Bay (Raoul Island), which was formed during the 2006 eruption. We’re now getting data back from Raoul Island, which will help us monitor any changes.”
Predicting future aftershocks
GNS has updated the aftershock forecasts for the East Cape earthquake.
Last Friday the probability of one or more 5-5.9 magnitude aftershocks was estimated to be greater than 99 percent (extremely likely). That had decreased to an estimated probability of greater than 85 percent (very likely), GNS said.
The probability of one or more aftershocks in the 6-6.9 range had decreased from 50–75 per cent (likely), to 20–35 per cent (unlikely).
As at 1pm Friday, the GeoNet network had recorded nearly 1700 further earthquakes in the East Cape and Kermadec regions following the mainshock events last Friday.
“The continued seismic activity that we’re seeing is very much in line with what we’d expect to see following a significant earthquake, like the events last Friday,” earthquake physics and statistics team leader Annemarie Christophersen said.