Magnitude 6.3 earthquake not on Greendale Fault
Preliminary analysis of seismograph records of the devastating magnitude 6.3 earthquake on Tuesday show that it was just 9km from the centre of Christchurch on a buried fault oriented roughly east-west. It is quite likely that the orientation of the fault results from a combination of both a pre-existing weakness in the rock and stresses resulting from the weight of the volcanic hills of Banks Peninsula. This is quite a simple scenario compared to the multiple faults that ruptured during the mainshock in September 2010.
The aftershocks that follow a large earthquake are initially located mainly on the periphery of the fault that ruptured during the main earthquake. During an earthquake, rocks slip past each other along a fault, producing earthquake waves that cause the ground shaking that we feel. Stresses (or pressures in the rock) essentially ‘pile up’ at the edges of the area that slipped, creating extra stresses that then start to trigger aftershocks. Over a period of months, the zone of aftershocks tends to spread out as stresses within the earth are redistributed. These aftershocks are themselves earthquakes, many of which occur on small unknown faults near to the original fault that ruptured.
This is the pattern we have seen with the aftershocks in Canterbury. Stress calculations show that the September mainshock increased stresses on east-west oriented faults in the Christchurch area. This is why there has been a migration of some of the aftershocks towards the east, which has unfortunately brought them much closer to the city. This, and the fact that some of them are very shallow, is why some have been felt so much more strongly than the September mainshock and in this case caused severe devastation.
There is no obvious structure directly connecting the faults that ruptured in the September’s magnitude 7.1 earthquake with the fault that generated the magnitude 6.3 event. On the contrary, precise aftershock relocations suggest that at least two north-east/south-west trending faults lie between the two and that there is no evidence from the earthquake data of an extension of the Greendale Fault.
The magnitude 6.3 earthquake is the largest aftershock to date and has been sufficiently large to generate its own aftershock sequence. In the first 24 hours, this additional aftershock sequence has been very active with four events greater than magnitude 5, the largest two being 5.9 and 5.7. These aftershocks have been strongly felt because they are all close to the city and are shallow. There may well be more aftershocks with magnitude 5.0 or greater in the next few days.
The frequency of aftershocks decreases with time, rapidly at first, but much more gradually later. When viewed over periods of weeks, this reduction tends to be fairly regular, but there are often anomalies, as the magnitude 6.3 event has shown. In time the rate of activity will decay back down to the level before the magnitude 6.3 event and then continue to decrease as before. Scientists use the observed earthquakes to refine the estimates of the likely occurrence of future aftershocks.
Graphic showing location of main shock, aftershocks above magnitude 3, and fault ruptures in Canterbury. Graphic by Rob Langridge and William Ries, GNS Science.