Six hours, three quakes, three tsunami warnings

Wow, what a day here in New Zealand!

At 2.27am, I was abruptly woken by a deep rolling quake that lasted over a minute. Normally, noticing such quakes in Auckland means a much larger earthquake has struck somewhere within the usual seismic hotspots of New Zealand. My Geonet app buzzed shortly after, indicating that indeed a M7.3 earthquake had struck east of the North Island. Felt reports were recorded across the length of the country. Shortly after a tsunami warning was issued for large areas of the North Island with mandatory evacuations. Smaller waves were recorded around the East Cape and at Great Barrier Island. Shortly after 5am, the threat had passed.

Just as I started snoozing off again, the phone buzzed again. Surely this was just a smaller aftershock? Nope! A M7.4 had just struck near the Kermadec Islands. It was too far away to notice, but had authorities scrambling again, issuing a second tsunami threat. Then at 8.28AM, a mammoth M8.1 struck, approximately 60km to the east of the first Kermadec earthquake.

This was a unique event and cause for concern. Generally aftershocks steadily decrease in magnitude and frequency. This was significantly more powerful than the first. At this stage, the first M7.4 was likely a foreshock of the second M8.1 earthquake. It was also only 19.4km deep which was very unusual. The earthquake was the result of reverse faulting in the Tonga-Kermadec subduction zone and such events typically occur at lower depths. Of the 29 7M+ earthquakes recorded in the Tonga-Kermadec subduction zone, 24 have struck deeper than the one today.

Just as two tsunami warnings had passed, another, much more serious one was issued covering most of the country, with specific land threats in place for Northland the the East Cape areas. And so at least tens – if not hundreds of thousands of people – evacuated for high ground. Aucklanders were warned from any coastal areas with police patrolling local beaches.

Over many hours, while we waited patiently, reports of tidal disturbances and unusual swells come in. Parts of the Kaipara Harbour had emptied out so extensively, normally unseen seabed was uncovered. Areas that were at low tide had changed to high tide within the space of an hour. A significant wave was captured at Tokomaru Bay (see the header image), but thankfully that was about it. Shortly after 1pm, the threat was over and we were all safe to return home.

Regular M5-6 aftershocks continue in both locations. The most likely scenario from here is that further aftershock earthquakes of lower magnitude will occur, but will decrease in frequency over the next 30 days.

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