Japan was rocked by a series of earthquakes today about 100 miles (160 kilometers) from Tokyo that injured two, cut off power to some 2,100 homes, and left the country on high alert for possible aftershocks. The largest quake hit at 1:45 a.m. local time in the Pacific Ocean off the coast of the Ibaraki Prefecture and measured 6.8 on the Richter scale, but Japan's meteorological agency told the Agence France-Presse (AFP) that it does not expect the tremors to result in a tsunami.

No major damage has been reported, and the two injuries are not believed to be serious. Highway officials lowered the speed limit as a precaution, although they had not detected any cracks in roads from the quake, which struck offshore at a depth of 25 miles (40 kilometers), according to the U.S. Geological Survey (USGS). The Japan Broadcasting Corp. reported that a second quake with a magnitude of 5.3 shook the same area about 30 minutes later, according to the Associated Press.

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 and, as time goes by, the movement of the plates causes the rocks around the fault to bend and stretch. This causes the rocks to behave like compressed springs, storing energy until the friction across the fault is not able to hold the rocks back, and a crack is created 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.

Japan, which lies at the crossing of four tectonic plates, is the site of 20 percent of the world's major temblors. In July, a 6.8-magnitude quake killed 11 people and shut down Tokyo Electric Power Co.'s Kashiwazaki-Kariwa nuclear power plant, the world's largest. In 1995, the 6.9-magnitude Great Hanshin Earthquake, also called the Kobe Earthquake, left more than 5,500 dead and more than 26,000 injured and caused more than $200 billion in damages. Few residents, however, remember Japan's most devastating earthquake of last century, the Great Kanto Earthquake of 1923, which had a magnitude of 8.3 and killed about 100,000.

Yesterday, there was a small 1.8 magnitude earthquake in the northern Virginia city of Annandale, 13 miles (20 kilometers) from Washington, D.C., that residents felt but was not strong enough to cause any damage, according to the USGS. The largest recorded earthquake in Virginia occurred on May 31, 1897 near Giles County in the southwest part of the state and measured 5.8 on the Richter scale.

Although the Richter scale has no upper limit, the larger earthquakes it measures have magnitudes of 8.0 or higher. More 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.