Kaikoura earthquake update: Magnitude revised

6pm, Wednesday 16/11/2016

We have revised the initial magnitude of the Kaikoura earthquake from M7.5 to M7.8. The revision is based on further information that we have collected over the last 48 hours since the earthquake.  

What has changed since our initial review

Based on our findings and in discussion with international researchers, early indications are that this is 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 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, we’ve gathered data from our entire network of seismic stations. All of these stations would not normally need to be included in magnitude estimates.

Further, our techs at GeoNet went out to several sites which we lost communication with and we have now been able to upload this information, so we have a more complete understanding of the ground deformation and strong-motion data.

Finally, our science teams have been working tirelessly, going up and down the affected areas and measuring the length of faults and how much they moved.  Their efforts have provided us with a clearer picture as to the size and length of the ruptures.

Based on all these 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. 

What this means

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.

Our 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.

We are in the process of revising our probabilities and scenarios based on this new information and should have this released within the next 24 hours.

Good science takes time

Our GeoNet seismic network is robust and records tremors and shakes throughout the country. However, with these very rare large events, it requires time and thought as to what all of this new data means. This earthquake required us to take a different approach and we have been triaging the data to reconcile all the different data sets. 

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