Cameron Smith, of The Terrace School, Alexandra, asks :-

What is difference between an earthquake and an aftershock? How do scientists know the difference from data recording?

Terry Webb, a geophysicist with GNS Science, responded.

I need to make it clear from the outset that aftershocks are just smaller earthquakes that occur after a large earthquake.

It may help your understanding of this if we think about the process that leads up to a large earthquake. As you probably know, the surface of the earth is made up of ‘tectonic plates’ which move slowly relative to each other. In New Zealand we sit astride the boundary of the Australian and Pacific plates, which move at roughly 40mm per year. Over hundreds to thousands of years this movement builds up forces in the earth’s crust, similar to stretching a rubber band. The equivalent of breaking a rubber band is an earthquake, where rocks fracture and move past one another on a fault, thus relieving the forces that built up in that location.

However, around the edges of the area of fault that failed, the forces built up over such a long period are not completely released in the initial earthquake. A period of readjustment occurs over days, weeks, or (following a large earthquake) years. This period of readjustment is accomplished through numerous small earthquakes which we call aftershocks.

Your question also asks why another earthquake nearby could not have been an independent earthquake in its own right. Usually aftershocks occur in a reasonably well-defined zone around the main shock and, as mentioned above, may continue for a long time. The rate at which aftershocks occur, however, drops off rapidly. We thus call a later earthquake an aftershock if it lies within the zone where many other small earthquakes (themselves aftershocks) are still occurring.

Over the past decade seismologists have come to understand how large earthquakes (say magnitude 6 and above) can trigger other large earthquakes over distances of tens of kilometres and in timeframes of minutes to decades. In this case the later earthquakes are triggered by small changes in the forces within the earth’s crust. Such triggered events usually lie outside the aftershock zone and so are caused by a slightly different process from aftershocks.