The powerful earthquake that struck Northern California on Thursday triggered a brief tsunami warning that affected about five million people along a stretch of the West Coast – from Northern California to Oregon – before it was lifted. Here are some things you should know about tsunamis:

What is a tsunami?

A tsunami, a word that comes from the Japanese characters for harbor and wave, is a series of extremely long waves caused by the sudden shifting of the ocean, according to the National Weather Service.

“Most tsunamis and the largest occur from earthquakes on reverse faults. These tsunami-generating earthquakes occur primarily in subduction zones, where tectonic plates collide and one is pushed under the other,” the National Oceanic and Atmospheric Administration said.

If it is large enough and closer to the seafloor, the energy from the earthquake can cause the seafloor to suddenly rise or sink. “This sudden vertical shift of the seafloor typically triggers a tsunami,” NOAA said.

“Tsunamis radiate in all directions from the point of origin and can spread across entire ocean basins,” the service says.

If the waves reach land, they can cause coastal flooding and the strong currents can last for several hours or days.

The Weather Service calls a tsunami “one of the most powerful and destructive forces of nature.”

How often do they occur?

According to the Global Historical Tsunami Database, tsunamis that cause damage or death near earthquakes occur about twice a year.

A tsunami that races across an ocean and causes damage or death more than 1,000 kilometers away occurs about twice a decade.

Where do they occur?

Tsunamis can occur in any ocean, sea or large body of water. However, certain areas are vulnerable to tsunamis due to their proximity to the source and other factors such as the depth and shape of the seabed.

According to the database, there were 754 confirmed events between 1900 and 2015. Of these, 78% were in the Pacific Ocean. According to the database, Japan has had the most tsunamis since 1900, followed by Russia and Indonesia.

A magnitude 9.2 quake, the second strongest earthquake ever recorded in the world, caused extensive damage in the Anchorage area in 1964 and the ensuing tsunami killed 131 people, including some in Oregon and California.

Most devastating tsunamis

In 2004, a magnitude 9.1 earthquake struck off Sumatra, an Indonesian island in the Indian Ocean. It triggered a tsunami up to 51 meters high, causing an estimated 230,000 deaths.

Another magnitude 9.1 earthquake struck Japan in 2011, triggering a tsunami with waves of up to 127 feet (39 meters). The tsunami claimed around 18,000 lives and triggered a nuclear power plant accident.

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