Quake alerts wrong message

By Mike Dinsdale

Whangarei hasn't been rocked by earthquakes in recent weeks, despite alerts being sent out saying it has. An anomaly in seismic sensors is to blame.

In the past 10 days three reports of minor earthquakes off Whangarei have been sent out by the international Earthquake Report website, the latest yesterday morning.

That alert said a 3.6 magnitude quake was felt at 8.56am at a depth of 254 km, 45km south-west of Whangarei. It was the fourth such report far this year.

GNS Science seismologist Caroline Little said Whangarei wasn't getting the shakes, but there was an anomaly in the seismic testing network.

The seismic sensor network around northern New Zealand was fairly sporadic compared to the south and the shock from earthquakes travelled in primary waves (P-Waves) and secondary waves (S-Waves).

P-Waves travel fastest and were usually felt first, while S-Waves were much slower, but more powerful, than P-Waves.

The sensors were picking up S-Waves from a seismic event much further away and interpreting them as P-Waves, then the automated system was sending out the alerts, she said.

The events were said to be off Whangarei as that was the closest centre to the relevant sensors, Ms Little said.

"It definitely wouldn't have been felt at all [in Northland] and it's completely wrong to say it was off Whangarei, it's an anomaly in the system."

Northland, along with South Otago, has the lowest earthquake risk in New Zealand.

No active faults are mapped within the Northland region and the whole Northland peninsula has been generally regarded as tectonically stable.

Since 1960, Northland has had eight earthquakes with a magnitude greater than 3.5.

The most recent earthquake activity in Northland was measured near Takou Bay in September 2006, with the largest of the small swarm measuring 3.4 and felt by residents in Kerikeri and Russell.

The aftershock that hit Christchurch on February 22, 2011, destroying the central city and claiming 185 lives, had a magnitude of 6.3

- Northern Advocate

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