Why is it an aftershock not another earthquake?

Because the aftershocks are caused by other parts of the fault line adjusting after the main earthquake

An aftershock is another earthquake. It's just referred to as an aftershock because they come after the initial large shock. Foreshocks come before. However, seismology is not an exact science. When large earthquakes like the recent one in Haiti occur, they generally occur in "earthquake swarms." In the end, seismologists often have to go backwards and look at their data to determine fore-, main- and aftershock. Occasionally, foreshocks and aftershocks will be classified as such, but can actually be larger than the main shock. But when it does happen, the seismologists "recompute" their data, and designate the new main shock as... the new main shock, forcing previous aftershocks to become foreshocks.

This means that it may take days or weeks to gather sufficient data to pinpoint the exact time, place, and magnitude of the major rupture that created the earthquake. This is one very good reason that we have absolutely no idea how to predict an earthquake at the moment. To be specific, we can't say, "The Big One" (no such thing) is going to happen on March 4, 2010. We can say, "Southern California will have dozens of 1- and 2-magnitude quakes every single day, a few 3-s, and a somewhat rare 4 that those of us who live down here are completely acclimated to."

But prediction? We can't even narrow it down to a month.

An aftershock is actually just a normal earthquake in every physical detail. Out of context, there is no way to tell the difference between any arbitrary earthquake and an "aftershock". The only real difference between the two is that an aftershock follows closely in the wake of a larger earthquake, and in roughly the same location as its predecessor. That larger, initial earthquake is usually referred to as the "mainshock".

there is not any distinction between an "earthquake" and an "aftershock". An aftershock is in simple terms a weaker earthquake that follows the unique seismic journey. The epicenter of aftershocks are actually not continually in a similar area with the aid of fact the unique quake. Aftershocks are smaller quakes that happen with the aid of fact the displaced fault shifts to house the outcomes of the biggest marvel. Had the 2d quake been extra beneficial than the 1st, it could have been denoted with the aid of fact the "earthquake", and the unique you are able to have been relabeled a "foreshock".