A late-afternoon earthquake hit the southern Japanese island of Okinawa, about 900 miles (1,400 kilometers) from Tokyo, the U.S. Geological Survey (USGS) says. About an hour and a half later, the USGS also reports, an earthquake shook Peru, 450 miles (725 kilometers) southeast of Lima.

Don Blakeman, an earthquake analyst at the USGS, told ScientificAmerican.com that the two Pacific Rim tremors are probably unrelated. "We don't really see evidence for one triggering the other at those distances," he explains.

The Japanese temblor measured a 6.0 on the Richter scale and struck at 4:42 local time 100 miles (165 kilometers) northeast of Naha, Okinawa's capital city. There were no reports of casualties or injuries. This is the third earthquake to strike the country since May 7, when a magnitude 6.8 tremor hit the largest Japanese island Honshu; six people were "slightly injured," according to the USGS. Honshu was shaken again on June 14, this time with a magnitude 6.9 temblor that killed at least 12 people.

The Peruvian tremor measured magnitude 6.2 and hit at 4:13 a.m. local time near the Andean town of Chuquibamba about 150 miles (240 kilometers) from Peru's border with Bolivia and Chile. According to officials, there was one casualty: a 93-year-old man died when a wall in his home collapsed on him. The quake hit the country less than a year after a magnitude 7.9 temblor ripped through the coastal province of Pisco, about 165 miles (265 kilometers) south of Lima, killing more than 500 people and injuring at least 1,000.

According to the USGS, today's tremor in Peru was more than 40 miles (65 kilometers) underground. Shallow earthquakes, such as the deadly one that struck China's Sichuan Province in May, tend to do more damage than do deeper ones near their epicenters.

Earthquakes are initiated by the release of energy stored in rocks clustered around a fault, which separates masses of Earth's crust known as tectonic plates. These rocks are held in place by friction. As time passes, the movement of the plates causes the rocks around the fault to bend and stretch. This causes the rocks to act like compressed springs, storing energy until the friction across the fault is no longer able to hold them back, and a crack forms across the fault as it begins to slip. This releases part of the built-up energy, some of which creates the seismic waves that travel to the surface and cause damage.

Earthquakes stop when there is not enough energy to keep them going. The energy released by the sliding fault needs to be substantial enough to break the friction holding rocks in place. In some cases, rather than slipping, the two sides along a fault rub together, which can cause a destructive, high-speed quake.

Although the Richter scale has no upper limit, the most devastating earthquakes have magnitudes of 8.0 or higher on its numerical ranking system. Moderate quakes register at 4.5 or greater in magnitude, whereas those of 2.0 or less are typically referred to as "microearthquakes," according to the USGS. American seismologist Charles Richter (1891 to 1989) developed the eponymous scale in 1935 to quantify earthquake magnitude, or strength.

Earthquakes often trigger other earthquakes, aside from just aftershocks. And high-magnitude quakes can cause smaller tremors thousands of miles (kilometers) away from where they strike. Likewise, imperceptible tremors—so-called "silent earthquakes"—can occasionally kick off a chain of events that result in tsunamis or larger temblors. Even knowing the risks that follow any sort of seismic incidence, scientists continue to have difficulty predicting when such natural disasters will strike.