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 surprising, 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 earthquake?
The Earth's crust is made up of several mobile plates that "float" on the layers below them. When two of these plates press together, there is a buildup of energy. When this energy is released at a fault, the ground shifts and we feel an earthquake.
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. The way scientists define aftershocks is simple: When there is a series of earthquakes, the largest one is called the "main shock," any before it are called "preshocks" and any after it are called "aftershocks."
How big can the aftershocks get?
"Large earthquakes have more, larger aftershocks — aftershocks can get as large as one magnitude below the main shock," says Sevilgen. "The basic rule is if you have one magnitude 9 earthquake, you can have one magnitude 8, 10 magnitude 7s, and 100 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.
If aftershocks can occur years after the fact, how do scientists distinguish an aftershock from a "new" earthquake?
If there is earthquake in Japan in a few years, it will be hard to tell whether it was triggered by the March 11 quake or had some other cause. Plus, a geophysicist's definition of aftershock doesn't have to do with causality — it has to do with timing and statistics. Scientists will look at a given time period before the main shock and after the main shock. If there are more earthquakes in the time period after the main shock, chances are some of those earthquakes are aftershocks.