When will they end?

Keith Lynch in The Press reports:

Christchurch has been rattled by its 10th aftershock in 24 hours after a magnitude 4.4 quake struck this morning.

GNS Science said the quake hit at 2.12am and was centred 10km south-west of the city at a depth of 7km. There were no initial reports of damage.

A flurry of quakes hit Christchurch yesterday, including a magnitude 5.1 shake at 6.03am and a magnitude 3.9 aftershock at 11.10am.

GNS Science seismologist John Ristau warned people to expect aftershocks of up to magnitude 5.0 every month to six weeks.

He said aftershocks could go on for “months, a year or maybe even over a year”.

GeoNet had recorded more than 3900 aftershocks since the magnitude 7.1 quake on September 4, he said.

I’m not sure what the data is for other quakes, but the sheer number of aftershocks plus the length of time (almost five months) they extend out for seems very high.

Omori’s Law which predicts frequency of aftershock’s says:

According to these equations, the rate of aftershocks decreases quickly with time. The rate of aftershocks is proportional to the inverse of time since the mainshock. Thus whatever the odds of an aftershock are on the first day, the second day will have 1/2 the odds of the first day and the tenth day will have approximately 1/10th the odds of the first day (when p is equal to 1).

10 aftershocks 140 days after the main quake falls well outside the norm I’d say.

So how long could the aftershocks last? Well the 1811-12 New Madrid earthquakes are still producing aftershocks!

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