A powerful 7.8-magnitude earthquake struck off the coast of Alaska, south of the Aleutian Islands, Tuesday night (July 21) local time, prompting fear of a tsunami, the U.S. Geological Survey reported.
The earthquake hit at 10:12 p.m. local time (2:12 a.m. ET on July 22, or 06:12 UTC) about 65 miles (105 kilometers) southeast of Perryville, Alaska, and 528 miles southwest of Anchorage. The tsunami warning, which had been issued for areas in south Alaska, the Alaska Peninsula and the Aleutian Islands, was later canceled.
The shaking happened when one block of Earth's crust on the seafloor slid on top of another, in a process called thrust-faulting (opens in new tab). In this case, the sliding happened on or near a subduction zone where the Pacific plate is slowly diving beneath (or subducting) the North American tectonic plate, according to the USGS. Thrust-faulting events similar to today's Alaska quake typically happen over an area about 75 miles long by 31 miles wide (120 by 50 kilometers), the USGS said.
The entire area is called the Alaska-Aleutian subduction zone (opens in new tab), where earthquakes are relatively common. Since 1900, six other earthquakes of at least a magnitude 7 have struck within 155 miles (250 km) of today's event. The largest of these, a magnitude-8.2 earthquake, struck on Nov. 10, 1938, at almost the exact location of today's quake, the USGS said. The second-largest temblor recorded by modern seismic instruments happened in this subduction zone (but farther away from today's quake), triggering a magnitude-9.2 earthquake on March 27, 1964; that quake produced a small tsunami, but its remote location meant there was little impact on people or infrastructure in the area, the USGS said.
Today's quake is considered shallow at about 17 miles (28 km) deep. "Anything below 70 kilometers is considered a shallow quake," CNN meteorologist Allison Chinchar previously said. "That's important, because shallow earthquakes often cause the most damage, compared to the ones that are deeper, regardless of the strength."
Earthquakes with a magnitude above a 7.6 and that are also shallow and happen due to thrust-faulting are more likely than other types of quakes to trigger tsunamis, according to the USGS. Today's earthquake would be considered a shallow one, as the rupture occurred about 6 miles (10 km) deep.
Originally published on Live Science.