A day after a powerful earthquake along the Pacific Ring of Fire hit Mexico, another large quake hit the same area of seismic activity on its opposite side, across the Pacific Ocean, on Wednesday: a magnitude 6.1 off the coast of Japan.
Wednesday's quake hit at 4:37 p.m. local time, or 8:37 a.m. PT, according to the U.S. Geological Survey. Its epicenter was about 200 miles off the coast of Japan, and about 315 miles northeast of Tokyo.
The quake hit about 228 miles from Fukushima, the site of the nuclear power plant that was inundated following a tsunami triggered by a magnitude 9.1 earthquake on March 11, 2011. That much more powerful quake 6 1/2 years ago was much closer to shore.
On Tuesday, a magnitude 7.1 quake hit in the state of Puebla, Mexico, on the anniversary of a 1985 quake that killed some 9,500 in the same region. The death told climbed past 200 throughout the day, and rescue efforts were ongoing Wednesday.
The Japanese quake Wednesday did not appear to cause any damage, but its location and size are a reminder of the volatile nature of the so-called Ring of Fire.
The 40,000-kilometer (25,000-mile) Pacific Ring of Fire stretches from the boundary of the Pacific Plate and the smaller plates such as the Philippine Sea plate to the Cocos and Nazca Plates that line the edge of the Pacific Ocean.
About 80% of all earthquakes strike in the Ring of Fire, according to Hongfeng Yang, a seismologist at the Chinese University of Hong Kong.
People are most at risk in Chile, Japan, the U.S. West Coast, and other island nations including the Solomon Islands to the western seaboard of North and South Americas.
They are at risk because they lie on subduction zones that are locked so tectonic energy must be released by large earthquakes.
How did the Ring of Fire form?
Tectonic plates are massive slabs of the Earth's crust. These move constantly above the mantle -- a layer of solid and molten rock below the Earth's crust.
The Ring of Fire was formed as oceanic plates slid under continental plates.
Volcanoes along the Ring of Fire are formed when one plate is shoved under another into the mantle -- a solid body of rock between the Earth's crust and the molten iron core -- through a process called subduction.
Large earthquakes -- which risk triggering tsunamis -- also occur in subduction zones.
How are earthquakes triggered?
Earthquakes represent the energy release from the interior of the Earth, where huge amount of heat is stored.
The heat drives the plates to move. When two plates move against one another and produce friction, it causes energy to build up. When the energy is released it triggers an earthquake.
"It takes tens of thousands of years for the energy to build up, but only a matter of seconds for it to be released," said Hongfeng Yang, a seismologists at the Chinese University of Hong Kong.
Tectonic plates usually move an average of a few centimeters each year, but when an earthquake strikes, they can move several meters per second.
Can earthquakes be predicted along the Ring of Fire?
As with the Sept. 7 earthquake off the west coast of Mexico, seismologists can't yet predict when or where earthquakes will strike, or how large they will be.
Some researchers argue that there are certain conditions -- such as hydraulic fracturing when we drill deep into the sea to extract energy resources -- induce earthquakes. But there's no hard scientific evidence to back this up.