Why the 2011 Christchurch earthquake is considered an aftershock
The magnitude 7.1 earthquake that struck the Canterbury region on 4 September 2010 was caused by the rupture of a fault network deep beneath the Canterbury Plains. The rupture started at 10 km below the surface, and then broke open a 24 kilometre rent across the surface of the plains. In the days following the quake, geologists from the University of Canterbury and GNS Science mapped the location of the surface rent, which extended from Greendale to near Rolleston, and named it the Greendale Fault. Part of the energy of the main earthquake, however, was contributed by rupture along branching faults near the western end of the Greendale Fault, faults that didn’t reach the surface.
Graphic showing location of main shock, aftershocks above magnitude 3, and fault ruptures in Canterbury. Graphic by Rob Langridge and William Ries, GNS Science.
Ever since the 4 September earthquake, aftershocks have become a daily worry for the people of the Canterbury region. The blocks of land that move along a fault usually have irregular surfaces, and long after the main movement has occurred, small areas continue to shift and readjust, producing the smaller quakes known as aftershocks. These aftershocks usually continue for many months after a major earthquake. Movement along a fault also places additional stress on the ends of the fault that haven’t moved. Over time, aftershocks may expand into adjoining areas along minor faults in response to this stress.
Since the initial earthquake on 4 September, scientists from GNS Science and Victoria University of Wellington have used an array of instruments to study the patterns of aftershocks. In addition to the national network of seismic recorders, many temporary recorders were deployed. Ground acceleration is recorded by strong-motion instruments of both the national network and the CanNet array. The CanNet array was originally developed by the University of Canterbury to record movement from a potential Alpine Fault earthquake, and it is now operated by GNS Science as part of the GeoNet project.
At first the aftershocks were clustered largely along the east-west faultline across the Canterbury Plains, but they soon spread well beyond the visible ends of the Greendale Fault. Over many months a cloud of aftershocks has developed, indicating a network of subsurface faults. One cloud of aftershocks extended both north-northeast and south-southwest from the eastern end of the Greendale Fault. At the south end of the zone was another line of aftershocks, roughly parallel to the Greendale Fault but many kilometres further southeast. It extended eastward into southern Christchurch and beneath the Port Hills area.
Aftershock activity on these adjoining hidden faults began soon after the 4 September earthquake. For example, on September 8th, only four days after the main 7.1 earthquake, an aftershock of magnitude 5.1 was felt strongly in Christchurch, causing further damage to earthquake-weakened buildings. This aftershock occurred near Lyttelton and was centred in nearly the same location as the magnitude 6.3 earthquake on 22 February. On Boxing Day, aftershocks centred close to the city’s central business district abruptly ended a busy shopping day and caused even more damage.
On 22 February 2011 another length of fault ruptured at the eastern end of the aftershock zone. This fault cut through the bedrock underlying the volcanic rocks of the Port Hills, but the break did not reach the surface. The rupture was less than five kilometres from Christchurch’s central business district, and generated a powerful magnitude 6.3 earthquake with unusually violent ground movements. The earthquake brought down buildings, caused liquefaction in large areas of Christchurch and triggered numerous rockfalls on the Port Hills.
Even though it occurred more than five months after the initial earthquake, scientists consider this damaging earthquake to be an aftershock, as it was caused by a fault rupture within the zone of aftershocks that followed the earthquake on 4 September 2010. The 22 February earthquake was extremely unlikely to have occurred without the preceding magnitude 7.1 earthquake and its other aftershocks. Other large New Zealand earthquakes have also had major aftershocks on completely different faults from the main shock. For example, New Zealand’s deadliest earthquake was the magnitude 7.8 Hawke’s Bay (Napier) earthquake on 3 February 1931. Its largest aftershock was a magnitude 7.3 quake that occurred about ten days after the initial earthquake. The epicentre of this aftershock was about 50 kilometres from the location of the main quake, offshore and to the east. It was described by some inland communities as even more violent than the February 3rd shock.