Incredibly, 20 Quakes of Magnitude 6.0 or Greater Rock Japan
Credit: NASA Earth Observatory.

The shaking in Japan didn't end with the country's largest-recorded earthquake.

Some 20 earthquakes of magnitude 6 or greater have rocked Japan since today's massive 8.9-magnitude earthquake struck near Honshu, according to the U.S. Geological Survey (USGS). The largest aftershock was a magnitude 7.1 quake that struck less than an hour after the main shock.

Nearly 100 earthquakes of magnitude 5 or greater (including the 20 large ones) have also followed the main shock. Each magnitude 5 earthquake is strong enough to knock a chimney down.

The number of aftershocks in Japan is not uncommon for an earthquake of this size, said geologist Eric Geist, of the USGS, at a news conference. [Image Gallery: This Millennium's Destructive Earthquakes]

The hits are expected to keep coming, said Tom Broker, also a USGS geologist.

"It's important to note that these aftershocks are going to persist for some time," Broker said in today's news conference. "We're going to be seeing large aftershocks for at least a year."

In February, a 6.6-magnitude aftershock ruptured near Maule, Chile — almost a year after what is now the sixth-largest earthquake in recorded history, a magnitude 8.8, hit in the same region.

A magnitude 7.1 aftershock isn't that unusual after an earthquake of this size. Because earthquakes are measured on a logarithmic scale, even an earthquake over magnitude 7 is still hundreds of times smaller than the magnitude 8.9 main event, said Terry Tullis, geologist and professor emeritus at Brown University. 

A shallow magnitude 6.2 earthquake recently hit western Honshu. It reportedly was not an aftershock, and it might be related to the main quake — but could just as easily be random, Tullis told LiveScience. "It might be that it would have just occurred anyhow. Magnitude 6 earthquakes are pretty common in this part of the world."

Honshu is Japan's largest island and is home to about 100 million people. The death toll in Japan a day after the earthquake and tsunami hit is an estimated 586, though that number could rise substantially with news that about 9,500 people are unaccounted for in the town of Minamisanriku in Miyagi Prefecture, according to news reports.

Brett Israel is a staff writer for OurAmazingPlanet, a sister site to LiveScience. Email Brett at bisrael@techmedianetwork.com. Follow him on Twitter @btisrael.

LiveScience senior writer Stephanie Pappas contributed to this report. Follow Stephanie on Twitter @sipappas.