Aftershocks: Why the Shaking Does Not Stop
After a big quake, the feed fills with smaller events for days or weeks. Here is what aftershocks are and how long they last.
Watch the live feed after any large earthquake and you will see a cluster of smaller quakes appear near the same spot. These are aftershocks, and they follow rules clear enough to forecast as a group, even though no single one can be predicted.
What causes them
A large quake shifts enormous blocks of rock, which changes the stress on the faults around the rupture. Some of those faults are pushed closer to their own breaking point. Aftershocks are those nearby faults adjusting to the new stress, releasing it in a sequence of smaller slips.
They follow predictable patterns
Two patterns hold remarkably well. Omori's law says aftershock frequency drops off roughly in proportion to time, so the day after a mainshock is far busier than a week later. Bath's law says the largest aftershock is typically about 1.2 magnitudes smaller than the mainshock. A magnitude 7 mainshock often produces a magnitude 6 aftershock, which is itself a damaging event.
How long they last
For a moderate quake, aftershocks may fade within days. For a great earthquake, they can continue for years at a slowly declining rate. The 2011 Tohoku sequence in Japan was still producing measurable aftershocks long after the mainshock. On the feed, this looks like a region that stays unusually busy well after the headline event.
Foreshocks complicate the story
Sometimes the first quake in a sequence turns out to be a foreshock, with a larger mainshock following. The problem is that a foreshock looks identical to an ordinary quake at the time. Only in hindsight does it earn the label. This is one reason no one can tell you a small quake means the danger has passed.
What it means for you
If you live in an active region and feel a strong quake, treat the following hours and days as elevated risk, not relief. The feed will usually confirm a swarm of aftershocks. Securing heavy objects and reviewing your plan during that window is time well spent.