The 6.7-magnitude earthquake that rattled Northern Japan on Monday struck just over 100 miles away from the epicenter of the devastating 2011 earthquake and tsunami that killed at least 15,890 people that year.
The close proximity isn't surprising. Monday's quake was technically an aftershock. That’s right—huge aftershocks to the devastating earthquake are still shaking Japan four years later, and they'll likely continue for some time. The one on Monday was, by the Wall Street Journal's count, the 830th aftershock of a magnitude 5 or higher since the March 2011 quake.
And there's reason to believe that significant follow-up temblors could continue to happen for years or longer.
Typically, the larger an earthquake is, the larger its aftershocks will be—bigger initial quakes also produce more aftershocks over time. Just how long might that be? "There are no hard and fast rules about this," Robert Williams, a geophysicist for the U.S. Geological Survey, told me.
But although aftershocks decrease in number as time passes, they don't necessarily decrease in intensity. Which helps explain why there was such a strong aftershock on Monday, years after the initial seismic event.
"The general public often believes that both the number and magnitude of aftershocks decrease with time, but it appears that only the number decreases," according to a 1997 USGS paper. "Thus, large late aftershocks are not uncommon." (For an earthquake to be classified as an aftershock, it has to occur one or two fault lengths away from the original, and, according to USGS, "during the period of time before the background seismicity level has resumed.")