How can an earthquake be classified as an after shock when it happens more than 20 days later???

Question

I just read that China had another "after shock" 5.3. How can be an after shock? Isn't a new earthquake??

Answer 1

No, because it has the same epicentre.ie it is further movement on the fault that caused the original earthquake.

If an earthquake triggers another earthquake in another location, that does not count as an aftershock.

Answer 2

With large earthquakes, aftershocks can occur for years. Here is a website that explains it in more  detail: http://pasadena.wr.usgs.gov/step/aftersh...