Why the aftershocks and when will they stop?

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Dr Mark Quigley, lecturer in Active Tectonics and Geomorphology in the Department of Geological Sciences at the University of Canterbury, explains why Canterbury is experiencing so many aftershocks.

Large cracks in the earth after the 7.1 earthquake. Aftershocks continue to rock Christchurch, Photo / Sarah Ivey
Large cracks in the earth after the 7.1 earthquake. Aftershocks continue to rock Christchurch, Photo / Sarah Ivey

My take on the aftershocks is as follows:

1. Aftershocks are caused by slip on the sticky parts of the fault and the fault tips that did not rupture during the main earthquake. They may also occur in the broader area around the fault, as the surrounding rock mass adjusts to the main earthquake.

It is also possible that some of these earthquakes may record slip on another fault, as large earthquakes such as our 7.1 are known to adjust the stress field in the crust and even trigger movement on other faults in the region. We know that other E-W trending faults are present throughout Canterbury, including offshore, so this is possible.

I am going out with a field team shortly to attempt to get a better understanding of the geometry of the main fault and possible others beneath the surface.

2. Significant aftershocks such as the ones we have been feeling in our beds at night or walking around during the day are likely to continue for weeks to months.

We will not get another Mw 7.1 earthquake on the same fault that generated this large earthquake as the main earthquake relieved most of the strain energy that had gradually built up on this fault over time.

An aftershock as large as Mw 6 is possible. However, my optimistic guess is that we are unlikely to get an aftershock as big as a Mw 6.

For instance, the largest aftershock following the 2010 Mw 7.0 Haiti earthquake was Mw 5.9 and occurred 8 days after the main earthquake. The largest aftershock following the 2010 Mw 7.2 northern Baja (Mexico) earthquake was a 5.7 and occurred 2 months after the main earthquake (Kevin Furlong, person commun).

Interestingly, the Baja earthquake also had a cluster of three Mw>5 aftershocks within an hour, similar to what we experienced here on the terrible night of September 6th. I know this is distressing a lot of you but I am hopeful that these aftershocks might be the biggest we get. However, they may not be, and we could get a bigger one months from now.

3. Although there is some uncertainty in the location of the aftershocks, it seems to me that the biggest ones have been located at the tips of the fault, near Rolleston (east end) and Greendale (west end). This makes sense if the slip gradient (differences in amount of slip along the fault during the main earthquake) is high near the ends of the fault, as we have been seeing in our fault offset measurements. In this case, the worst aftershocks we are likely to feel in Christchurch are likely to be the ones associated with slip on the eastern tip of the fault and in the region east of the fault, although this will also depend on the ground conditions and other properties of the aftershock earthquake, such as the depth.

A shalllow aftershock will feel more intense than a deep one. We recently felt one this morning (Wed Sept 8) that seemed quite intense. Geonet reveals that it was a Mw 5.1 within 5 km of Lyttelton at a depth of 6km. The reason it felt so intense here was because it was close and was shallow.

4. In general, both the magnitude and frequency of aftershocks will diminsh with time. So please try to remain as positive as possible - I know it is scary but a big part of the fear is your recollection of the shock you felt during the main earthquake.

Click here to read more from Dr Quigley.

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