How long does an earthquake last?
People who have just felt an earthquake often ask how long it lasted. What they are really wanting to know is how long they felt the shaking. This depends on the size of the earthquake and their distance from it, because earthquake waves spread out as they travel, but also become weaker. A magnitude 6 earthquake several hundred kilometres away can be often be felt for 30 40 seconds. The actual duration of slip on the earthquake fault is usually quite brief just a few seconds for a magnitude 6 for example. This is because the fault rupture spreads very quickly (at about the S wave speed), so the whole process of faulting is over very quickly. During the very largest earthquakes, fault rupture can continue for up to 5 minutes as the rupture spreads over a length of say 1000km. For these earthquakes very high levels of aftershocks mean that the ground can be felt shaking continuously for some hours.
