The above timeline shows the evolution of publicly available magnitudes for the M7.8, 13 Nov 2016 Kaikoura earthquake in New Zealand. The gray bar shows M7.8±0.1 magnitude units.
The magnitude estimates before about 10min are mostly less than M7.6, with the exception of Early-est Mwpd, which is always high at M8.1-8.2. These earliest estimates generally increase in time as more stations become available and as the seismographs record more of the complex, 2min long earthquake rupture, including the largest release of energy between 60 and 90sec:
– Source time function from SCARDEC
The form of this source time function is seen in the P wave seismograms for this earthquake, here station CASY in Antarctica at 41° distance:
After about 10min most magnitude estimates are within 0.2mu of the current accepted M7.8 for this event. These include W-phase Mww magnitudes, which usually capture the full energy release and magnitude of large earthquakes that can be approximated by a point (centroid) source.
On 16 Nov GeoNet revised the magnitude of the Kaikoura earthquake from M7.5 to M7.8.
GeoNet magnitudes: Determination of origins and magnitudes – SeisComP3 (pers. comm. J H Gurney @UKEQ_Bulletin)
http://geofon.gfz-potsdam.de/eqinfo and pers. comm. Joachim Saul GFZ Potsdam.