Tsunami warnings fading after one of the largest earthquakes ever recorded. Here's what to know

HONOLULU — One of this century's most powerful earthquakes struck off the coast of Russia and generated tsunami warnings and advisories for a broad section of the Pacific, including Alaska, Hawaii and the U.S. West Coast and as far south as New Zealand. Now warnings are being downgraded in most areas, though advisories remain in place as more aftershocks are possible. The quake registered a magnitude of 8.8 and was centered off the coast of the Kamchatka Peninsula in Russia's Far East, according to the U.S. Geological Survey. It struck early Wednesday local time, which was still Tuesday in the U.S. Here's what to know: What a tsunami is Tsunamis are waves triggered by earthquakes, underwater volcanic eruptions and submarine landslides. After an underwater earthquake, the seafloor rises and drops, which lifts water up and down. The energy from this transfers to waves. Many people think of tsunamis as one wave. But they are typically multiple waves that rush ashore like a fast-rising tide. "Tsunamis cross the ocean at hundreds of miles an hour — as fast as a jet airplane — in deep water," said Dave Snider, tsunami warning coordinator with the National Tsunami Warning Center in Alaska. "But when they get close to the shore, they slow down and start to pile up." It could take minutes for waves to hit land next to the site of a major quake. It could take hours for tsunamis to cross the Pacific Ocean. The speed of tsunami waves also depends on ocean depth. They travel faster over deep water and slow down in shallow water. People were urged to stay away from coastlines until any wave surges passed in places as far away as Fiji, Samoa, Tonga, Federated States of Micronesia and Solomon Islands. Some tsunamis are small and don't cause damage. Others can cause massive destruction. In 2004, a 9.1 magnitude earthquake off Indonesia caused waves that leveled remote villages, ports and tourist resorts along the Indian Ocean across Southeast and South Asia. Some 230,000 people died. A 9.0 magnitude quake and tsunami ravaged parts of Japan's northeastern coast on March 11, 2011, killing about 20,000 people and triggering a nuclear meltdown. The power of this quake and its effects The 8.8-magnitude quake was among the four strongest earthquakes this century, according to the USGS. It was also the sixth-biggest quake ever recorded, said Simon Boxall, a principal teaching fellow at the University of Southampton's Physical Oceanography Research Center. The regional branch of Russia's Emergency Ministry on Kamchatka warned that scientists expect aftershocks at magnitudes of up to 7.5. The earthquake occurred along the Pacific Ring of Fire, the ring of seismic faults around the Pacific Ocean where more than 80% of the world's largest quakes occur. Several tectonic plates meet there. The ring gets its name from the volcanoes that surround it. While not all earthquakes lead to tsunamis, this one generated a series of them spreading outward from the epicenter off the coast of Russia's Kamchatka Peninsula. "It's a bit like throwing a very, very large rock into the sea and then watching the waves propagate away from that rock, that splash,'' Boxall said. "And so that's what's happened in this case. And that's why this particular one has generated a tsunami. It's not huge. It's not one that's going to cause mass devastation. But it will cause coastal flooding and it will cause damage, and it does put lives at risk if people don't move to high ground.'' The effects of this earthquake so far A tsunami height of 3 to 4 meters (10 to 13 feet) was recorded in Kamchatka, while tsunami waves about 2 to 5 feet high reached San Francisco early Wednesday, officials said. Other areas have seen smaller waves. The International Atomic Energy Agency said initial reports showed there had been no safety impact on nuclear power plants along Japan's Pacific coast. Damage and evacuations were reported in the Russian regions nearest the quake's epicenter, and officials declared a state of emergency in several areas. Several people were injured, but none gravely, and no major damage has been reported. Additional aftershocks are possible, putting the entire Pacific Rim on tsunami watch. A tsunami warning remained in effect for parts of the northern California coast. Much of the Pacific coast of North America, spanning from British Columbia in Canada to down the U.S. West Coast and into Mexico was under a tsunami advisory. How tsunami warnings are issued In Hawaii, emergency authorities blast alerts to people's cellphones, on TV and radio and sound a network of sirens. In Alaska, some communities have sirens, and information is available on weather radio or public radio broadcasts. In the U.S., the National Weather Service has different levels of alerts: A warning means a tsunami that may cause widespread flooding is expected or occurring. Evacuation is recommended and people should move to high ground or inland. An advisory means a tsunami with potential for strong currents or dangerous waves is expected or occurring and people should stay out of the water and away from beaches and waterways. A watch means that a tsunami is possible and to be prepared. Kirka reported from London.