December 8, 2017 Tsunami: Kermadec Islands

An unusual event that affected the Kermadec region and northern New Zealand last Friday (8th Dec).

At 3.10pm local time (2:10 am UTC) there was an earthquake of about M6 about 200km south of Raoul Island in the Kermadecs, and over the next 2-3 hours there were unusual waves recorded on the Raoul Island and North Island tide gauges (see attachment). The wave periods are about 5 minutes, and the peak-to-trough amplitude is up to about a metre on Raoul Island and up to about half a metre on the North Island gauges. A reverse travel-time analysis seems to confirm that the source of the waves was coincident with the time and location of the earthquake. The amplitudes are large for an M6 earthquake, and it does not appear to be a slow ‘tsunami earthquake'. The location is on the Kermadec seamount ridge so a volcanic or landslide source are possibilities.

Most interesting is that the moment tensor solution is highly isotropic, which would be typical of an explosive source. So perhaps a volcanic event? On the other hand, my volcanologist colleagues point out that the earthquake does not appear to be part of a sequence, which would be more typical for an eruption, and there is no sign of a pumice raft or excess SO2.

(all above from reference #11965)

2017, Dec 08 02:09:43 UTC, 12.0 depth, -30.5548 -178.4924, 5.9 Mww. An unusually large tsunami for a quake of this size was recorded on Raoul Island and on the North Island of New Zealand.