The quake caused federal experts to issue a tsunami advisory for possible dangerous currents in coastal areas of California and Oregon.
The National Oceanic and Atmospheric Administration's West Coast and Alaska Tsunami Warning Center says an advisory means that a tsunami capable of producing strong currents or waves dangerous to persons in or very near the water is imminent or expected.
The center says widespread inundation is not expected.
The hazardous period will continue into early Wednesday morning.
"The southern exposed beaches are the most vulnerable since the earthquake was generated southwest," meteorologist Dan Gudgel said.
The rise in water is not expected to coincide with the high tide.
Warning center official Guy Urban says a tsunami of 12 inches to 26 inches has been measured since the earthquake. Urban says it would be dangerous to people swimming or on beaches near the water.
A tsunami did strike the capital of American Samoa.
Water flowed about 100 yards inland in the U.S. territory before receding, according to local reports. A tsunami warning was issued after the Samoa quake for coastal areas of California and Oregon, advising coastal residents to avoid the waters throughout Tuesday evening.
A local Samoan reporter said Tuesday evening that victims "are everywhere" in a local hospital -- and that the dead could possibly number up to 20. The reporter also confirmed three or four villages on the tourist coast near the town of Lalomanu on Samoa's main island of Upolu had been "wiped out" by the tsunami.
The Samoa islands are comprised of the nation of Samoa and the U.S. territory American Samoa, with a combined population of roughly 250,000. Experts predicted the number of casualties would rise throughout the evening.
Japan's Meteorological Agency also issued a tsunami warning all along its eastern coast.
The agency warned Wednesday morning of a possible tsunami up to about 1.6 feet (50 centimeters) tall.
The agency warned the tsunami could hit all along Japan's eastern coast, on an area ranging from Japan's northern island of Hokkaido to the southern tip of Kyushu island, with the Okinawa islands to the far south also included.