New Zealand’s GeoNet has revised their initial magnitude of the November 13, 2016 Kaikoura earthquake from M7.5 to M7.8. The agency added that early indications show this was one of the most complex earthquakes ever recorded on land.
“This complexity means we have had to take extraordinary efforts to determine the magnitude, depth, and locations,” the agency said.
The very long time it took for the faults to rupture (over one minute) meant that the standard methods of calculating magnitude were insufficient to capture the full energy released.
Due to the size of the quakes, GeoNet gathered data from their entire network of seismic stations. All of these stations would not normally need to be included in magnitude estimates.
“Based on all ongoing efforts, we can say with some confidence that the earthquake was an M7.8. This is consistent with estimates from several other international agencies, specifically the USGS. Their early model provided us important information and we used all our additional data sets to confirm the magnitude.”
“The new magnitude just tells us what we think most people who felt the earthquake already know: it was powerful, and went on for a long time over a large distance. It doesn’t change what happened but it does provide us with more knowledge about how significant the event was.”
The recent analysis confirms the complexity of this event. It does not change any of the observations of strong ground motion, fault breaks or GPS recorded movement of the earth’s surface – these are physical observations independent of the magnitude of the earthquake.
Ground shaking caused by the Kaikoura Earthquake reached over MMI VIII near the fault rupture, on the Modified Mercalli Intensity Scale and is considered severe. In Wellington, some areas experienced shaking up to MMI VI-VII, which is considered strong. This is based on the current Shakemap calculation, which includes both measured data and estimated values.
GeoNet’s director, Dr. Ken Gledhill, said ‘it was a monster quake, one that has shocked us all with its intensity and ferocity. Because of its size, it made our world shake strongly but relatively slowly for a very long time.’
Gledhill briefly reflected on the supermoon connection and earthquake lights during the quake, saying that GeoNet just doesn’t have any way to measure their effects using existing sensors. “So, I am just going to leave that topic for other people to discuss.”