Ask An Expert: Understanding magnitude

21:07, Aug 23 2011
Will Ries, of GNS Sience
Will Ries, of GNS Sience, marks a trench dug across the Alpine fault near Inchbonnie.

You have said that the magnitude and energy released during an earthquake is related to the length of the fault. What is the measurement that gives you that information?

The September and February quakes are on fault lines that are incredibly short when compared with the Alpine Fault, and yet the magnitude commonly given for a major rupture on this fault is around 8.

Does that depend on how much breaks in one go, and could the whole fault line rupture in one event? If so, what could that mean for the South Island in terms of magnitude and devastation? - Sandra.

When comparing fault and earthquake size and the damage these cause, a number of things need to be considered.

Proximity to the fault and the size of the earthquake that occurs on the fault mainly dictates the level of damage.

The earthquake magnitude is not a linear scale. For every step up in magnitude there is about a 30 times increase in energy released. In other words, a magnitude-7 earthquake releases about 30 times more energy than a magnitude 6.


Fault length gets bigger with magnitude, but the factor is more like 10. We have seen that the magnitude- 6.3, February 22 earthquake was on a fault about 10 kilometres long. The magnitude-7.1 Darfield earthquake was on the Greendale Fault, of about 35km. So, the Alpine Fault quake rupture at magnitude 8 or so might be about 10 times longer than the Greendale Fault.

From field studies, we have some evidence that Alpine Fault ruptures might be of the order of 400km long, fairly consistent with the factor of 10 scaling.

The impact of an Alpine Fault magnitude-8 quake will be widespread - very damaging on the West Coast from Milford Sound to Hokitika, with lesser damage in the southern North Island, Nelson and Christchurch. - Dr Kelvin Berryman, manager, Natural Hazards Research Platform, GNS Science.

Can you tell me, please, what is the highest risk Canterbury faces? - Natalie.

I presume you mean risk from natural hazards, but it is important to consider other risks that we face every day.

Being out and about on the highways is by far a greater risk than natural hazards. With natural hazards the highest risk probably results from different things in different places (although we still do not know enough about all of the processes to make accurate comparisons).

In Kaikoura, tsunami and nearby quakes are events to be prepared for.

As we have seen in Christchurch, quakes pose a risk, but they are rare events on average.

In the Canterbury foothills and mountains, quakes causing rockfalls, river floods and avalanches pose significant risks. - Dr Kelvin Berryman.

Is it better to have frequent small daily aftershocks gradually releasing energy than to go for a day or several without any? When there are no aftershocks or few small shocks for several days, I often get an uneasy feeling that the tension might release as a larger shock. - Rob.

Aftershock sequences behave in relatively predictable ways.

With time, the quiet periods between felt aftershocks will increase. This reduction in the frequency is a good sign, meaning we are moving closer to the end of the aftershock sequence.

Statistically, the occurrence of small aftershocks has little influence on the likelihood of a large aftershock in the following few days. This is mainly because of the scaling nature of quakes.

As intuition would suggest, small quakes affect relatively small areas, whereas large quakes affect larger areas.

Consequently, the amount of stress released or transferred in a small quake is not usually sufficient to trigger a much larger quake, especially as the amount of transferred stress decreases rapidly away from the epicentre. - Dr Bill Fry, seismologist, GNS Science.

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