Every earthquake starts small. Beginning at one point, it extends outward, causing tremors in and around its path. At some point, though, all earthquakes stop. So what brings this mighty process to a halt?
That’s an important question, because the duration of an earthquake helps determine how much damage it will do. Will the tremor be of mild magnitude—say, a magnitude 4 on the Richter scale, the kind that occur all over the world every day—or a 9, which happens no more than once a decade on average?
To understand what makes an earthquake stop, it’s important to understand what makes it go: the release of energy stored in rocks clustered around a fault, which separates masses of the earth’s crust known as tectonic plates. These rocks are held in place by friction. As time goes by, the movement of the plates causes the rocks around the fault to bend and stretch. That, in turn, causes the rocks to store energy, like compressed springs. Eventually, friction across the fault isn't able to hold the rocks back, and a crack is created across the fault as it begins to slip. This releases part of the built-up energy, some of which creates the seismic waves that travel to the earth’s surface and cause damage.
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Earthquakes stop when there isn’t enough energy to keep them going. The energy released by the sliding fault needs to be enough to overcome the friction holding rocks in place. Once the earthquake comes up against too much friction, it will stop.
An earthquake can be stopped in its tracks, for example, when the crack on the fault hits material that doesn’t slide as easily—in other words, that has more friction. An analogy would be skiing off of snow onto dirt—you will stop pretty quickly. Hitting the site of a recent earthquake can also stop a tremor, because such areas don’t have enough built-up energy to propel an earthquake. A third possibility might be that the fault simply ends; the amount of energy required to cut a new fault into intact rock is far greater than the amount required to break an existing fault.
Then again, earthquakes can jump from one fault to another, often as far as 2.5 miles (4 kilometers) apart. So just as you never know when a big one will hit, it’s hard to say when it might stop.