Japan was rocked Thursday by a 7.1 magnitude earthquake just off its coast. Technically, that strong shaking was an aftershock of the devastating 9.0 temblor that hit the nation nearly a month ago. But it wasn’t a shocker, says Volkan Sevilgen, an earthquake researcher with the U.S. Geological Survey. He explains the science of aftershocks and why more shaking is sure to follow soon.

What is an aftershock?

Sevilgen says that whenever there is a large earthquake, there is a big change in stress along the fault line. That change triggers other, smaller quakes in the same region.

How big can the aftershocks get?

“Large earthquakes have more, larger aftershocks,” says Sevilgen. “The basic rule is if you have one magnitude 9 earthquake, you can have 10 magnitude 8s, 100 magnitude 7s, and a thousand magnitude 6s.”

Some earthquakes follow this pattern very closely; others are more random — it’s all a matter of probability. Since the March 11 earthquake, there have been about 400 aftershocks greater than magnitude 5, and three over magnitude 7. Sevilgen says the 7.1 magnitude earthquake reported on the morning of April 7 is no surprise.

How far away from the first earthquake can aftershocks occur?

Aftershocks typically occur near the fault line of the original quake. Sometimes, seismic waves can cause earthquakes far away. Sevilgen says geologists call these quakes “triggered events” if they are more than one fault line’s length away from the main shock.

When will the aftershocks stop?

“With an event like this, aftershocks can continue for years — maybe decades,” says Sevilgen.

The frequency of the aftershocks usually decreases over time. You might see dozens of large aftershocks in the first few hours after the main shock, but one month later, notable earthquakes only occur every few days. But this doesn’t mean they are getting smaller. It is still possible to have a 7.0 magnitude aftershock years after the main event. Copyright 2011 National Public Radio. To see more, visit http://www.npr.org/.