University of Canterbury Physics PhD student John Holdaway has calculated that the energy released by the M7.8 quake on 14 November accounted for more than 70% of the energy released in all New Zealand earthquakes of the past seven years combined – enough energy to power every home in the South Island for a year.
The M7.8 quake on November 14 accounted for more than 70% of the energy released in all New Zealand earthquakes of the past seven years combined. The energy released in two minutes during the M7.8 quake was around 32 quadrillion Joules – the equivalent of 8 million tonnes of TNT, or detonating 400 Nagasaki atomic bombs. To put that into perspective, the same amount of energy could power the city of Christchurch for three years – or every home in the south island for a year – or the whole of residential New Zealand for three months. It’s comparable to ten simultaneous Darfield quakes. The aftershocks alone from the first three days after the main event have already released another 800 trillion Joules – more energy than all New Zealand earthquakes in 2015.
This graph was made using publicly available magnitude data for tens of thousands of quakes over the past seven years from the GeoNet database. The energy released in a large earthquake can be estimated from its magnitude using a formula developed by Båth in 1966, and the relative energy of an aftershock can be derived by comparing it to the main quake using another equation. To make the graph, I converted the magnitude for each quake to an energy value in Joules, then combined these energy values across different years. What you see in the graph is the annual sum of the energy released by tens of thousands of individual quakes.
It’s worth noting that magnitude scale is logarithmic – a magnitude 5.2 quake is twice as powerful as a magnitude 5.0 quake, and a magnitude 6 quake releases 32 times as much energy as a magnitude 5 quake. Because of this, the second and third largest quakes in the 2010-2016 period – the M7.1 Darfield quake in 2010 and the M7.1 Te Araroa quake earlier this year – are the most visually apparent in the graph after the M7.8 quake. If the data for 2009 were included, the M7.8 Fiordland quake would approximately match the size of the M7.8 Kaikoura quake. We were fortunate the Fiordland quake was centred in a relatively remote area; we were not so fortunate with the most recent seismic activity.
The 22 Feb 2011 Christchurch quake is not particularly prominent on the graph, despite being very damaging in a small local region. This is again due to the logarithmic magnitude scale – at M6.3, it released about 180x less energy than the M7.8 quake. The reason that the Christchurch quake was so damaging despite its relatively small magnitude was because of the combination of several unfortunate factors. Firstly, the hypocentre was very close to a major city and situated at the extremely shallow depth of 5 km. The fault also produced an unusually high peak ground acceleration – 2.2 times the force of gravity, one of the highest values ever recorded for an earthquake. It also combined uncommonly violent vertical jolts with the more typical horizontal shaking. The combination of these factors was simply more than many buildings were designed to withstand.
GeoNet database with the publicly available earthquake data used for the graph: http://quakesearch.geonet.org.nz/
Quake energy release formula for large earthquakes (M5+) developed by Båth in 1966, where M is the magnitude of the quake: