On December 8, 2017, a M5.9 earthquake struck ~200km south of Raoul Island and on the southern margin of Curtis Island in the Kermadecs, New Zealand. A few hours after the seismic event, tunami waves – reaching up to 1 meter – were measured at the GeoNet‘s Raoul Island pressure gauges. Scientists are baffled: normally tsunami waves aren’t expected after earthquakes of this magnitude. Since then, the cause of the seismic waves and the tsunami has been a bit mysterious as first results indicate the event is unlikely to have been the result of movement on a tectonic fault (i.e. a typical earthquake) or an underwater landslide. Like very strange!

The following map shows the apparent source of the seismic waves (green star) and the surrounding pressure gauges that recorded tsunami waves (shown by the coloured triangles).

mysterious tsunami kermadec New Zealand, Mysterious tsunami waves detected in the Kermadecs, New Zealand on December 8 2017, Mysterious tsunami waves detected in the Kermadecs, New Zealand on December 8 2017 after a M5.9 earthquake
Mysterious tsunami waves detected in the Kermadecs, New Zealand on December 8, 2017 after a M5.9 earthquake: the apparent source of the seismic waves (green star) and the pressure gauges that recorded tsunami waves (shown by the coloured triangles). via GeoNet

The tsunami waves were first recorded by the Raoul pressure gauges 2-3 hours after the M5.9 earthquake:

mysterious tsunami kermadec New Zealand, Mysterious tsunami waves detected in the Kermadecs, New Zealand on December 8 2017, Mysterious tsunami waves detected in the Kermadecs, New Zealand on December 8 2017 after a M5.9 earthquake
Raoul pressure gauges recorded tsunami waves of more than 1 meter in magnitude after the M5.9 earthquake. Diagram shows tsunami arrival time (in minutes after 3.10pm) and height (cm). via GeoNet

A short time later, tsunami waves were also detected at various points around the North Island but at a much smaller scale. The locations of the pressure gauges are shown in the map above.

mysterious tsunami kermadec New Zealand, Mysterious tsunami waves detected in the Kermadecs, New Zealand on December 8 2017, Mysterious tsunami waves detected in the Kermadecs, New Zealand on December 8 2017 after a M5.9 earthquake
Smaller tsunami waves were then recorded by North Island pressured gauges showing tsunami arrival time (in minutes after 3.10pm) and height (cm). via GeoNet

The cause of the seismic waves and the tsunami has been a bit mysterious as first results indicate the event is unlikely to have been the result of movement on a tectonic fault (i.e. a typical earthquake) or an underwater landslide.

Since then event, baffled scientists across the world have been able to collect more information and discuss the different possible processes that generated the seismic wave and related tsunami:

  • A geothermal explosion (a steam and rock explosion underwater). This occurs when water is heated quickly, generating steam and as it expands it fractures the rocks.
  • A large (several metres), sudden, upward movement on a circular fault (these are commonly found at volcanoes).
  • An undetected submarine volcanic eruption may have trigger the seismic wave. Such an eruption would have expelled pumice or potentially SO2 gas emissions, and there are no known volcanic vents near the source of the seismic waves.
mysterious tsunami kermadec New Zealand, Mysterious tsunami waves detected in the Kermadecs, New Zealand on December 8 2017, Mysterious tsunami waves detected in the Kermadecs, New Zealand on December 8 2017 after a M5.9 earthquake
What is the cause of the mysterious tsunami wave that formed after a M5.9 earthquake in the Kermadecs, New Zealand?

Interestingly, an almost identical event occurred in 2009 at roughly the same location, and there have been similar events observed offshore Japan in 1984, 1996, 2006 and 2015.
GNS and international scientist are working towards better understanding what occurred and we will continue to update you with what they will learn from this event.

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