Difference Between An Aftershock And An Earthquake

[Lauren]

Sorry if this has been asked before, i had a quick look but couldn't find anything.

I know that aftershocks are considered quakes of a smaller magnitude following a larger quake.

But when do they stop becoming aftershocks and become an earthquake in their own right?

[Optimus Prime]

The way I thought these people distinguish between earthquakes and aftershocks are that an aftershock precedes the main earthquake and is always lower in magnitude and then an increase in magnitude would be an earthquake. For example;

6.0 earthquake

5.5 aftershock

5.1 aftershock

3.3 aftershock

2.7 aftershock

1.5 aftershock

5.0 earthquake

ect....

That's without looking on wikipedia. Might be totally wrong.

[Crepuscular Ray]

Aftershocks come after, rather than precede. Smaller ones before the main one are precursors, aren't they? If the figures I saw this morning were right, the first large one was a precursor to the second (although there is always the possibility that if they weren't on exactly the same fault they could both be classified as separate quakes rather than precursors or aftershocks).

[WaitingForMackerel]

Seems to me from what i have read that the correct categorisation of an aftershock/precursor cant be done untill after the event. The previous posts have got the right idea, if a given shock is after a larger one it is called an aftershock, if a small quake is followed by a larger one it becomes a precursor. Some times a shock might be considered the main quake only to be superseeded by a larger one and reclassified as a precursor. They would also have to occour in the same region of the main quake.

There is a statistical distibution of aftershocks called Omuri's law, Baths law and the Gutenberg-Richter law, but it would be incorrect to suggest that these are wholly accurate. They are statistical relationships that best match the recorded data rather that being based in the science of the earthquake zone.

Anyway it all seems pretty vague. What i am trying to get at is that no quake can be accurately classified at the time of the quake. It can only be done when following quakes have been recorded and it position in the sequence can be correctly described.

Its all on here

http://en.wikipedia.org/wiki/Aftershock

[Lauren]

That's what I suspected.

How close to the epicentre do the aftershocks have to be to be considered as aftershocks and not a seperate quake?

[WaitingForMackerel]

Distribution of aftershocks

Most aftershocks are located over the full area of fault rupture and either occur along the fault plane itself or along other faults within the volume affected by the strain associated with the main shock. Typically, aftershocks are found up to a distance equal to the rupture length away from the fault plane.

The pattern of aftershocks helps confirm the size of area that slipped during the main shock. In the case of the 2004 Indian Ocean earthquake and the 2008 Sichuan earthquake the aftershock distribution shows in both cases that the epicenter (where the rupture initiated) lies to one end of the final area of slip, implying strongly asymmetric rupture propagation.

To quote Wikipedia

[Optimus Prime]

Be careful when using wikipedia...just be careful lol.