As of 4:43 a.m., a tsunami warning is in effect for coastal areas of California after a reported 8.9-magnitude earthquake struck off the northeast coast of Japan on Thursday night, authorities said.

"In Santa Clara County we are monitoring the event but we don't anticipate any impact, but it's a situation that could change," said Kirstin Hofmann, director of emergency services for the county.

"We anticipate that while we've had the first influx of the tsunami, we'll be watching it during the day," she said. 

The National Weather Service said the initial waves could hit San Francisco at 8:08 a.m. Friday. The Great Highway will be closed from Point Lobos at 48th Avenue to Lake Merced, San Francisco Police Lt. Troy Dangerfield said. Residents are asked to avoid this area until further notice.

As of 3:44 a.m., BART reported that it was activating its Emergency Operations Center; as of about 4 a.m., no service changes were planned.  

As of 2:30 a.m., U.S. Coast Guard officials on Yerba Buena Island said they are not planning to take extra precautions, and that they don't expect boat traffic or ferry service in or around the Bay to be affected.

According to a 4:15 a.m. alert from the National Weather Service, "A tsunami was generated and is expected to cause damage to the northern California coast. Persons at the coast should be alert to instructions from local emergency officials."

Wave height forecasts are as follows: 2 feet for Fort Point in San Francisco; 2.3 feet for Pacifica; 2.6 feet for Half Moon Bay and 2.8 feet for Santa Cruz.

Learn more about arrival times for specific areas here.

"Due to rapidly changing conditions associated with tsunami wave activity... listeners are urged to tune to local emergency alert system media for the latest information issued by local disaster preparedness authorities. They will provide details on the evacuation of low-lying areas... if necessary... and when it is safe to return after the tsunami has passed," the National Weather Service noted.

A tsunami watch through the morning was issued shortly after midnight, forecaster Steve Anderson said.

Sea levels are expected to rise off the coast of California by less than a foot by 7:30 a.m. in response to the earthquake, he said.

Anderson advised coastal California residents to stay away from the ocean Friday morning.

The last tsunami to hit northern California did so in 2005, according to the National Geophysical Data Center. There were no reported fatalities. 

Tsunami alerts are based on the magnitude of the earthquake that triggers it. For earthquakes with a magnitude of 7.5 or above, a watch is put into effect for areas within three hours tsunami-travel time outside the zone where warnings have been issued, according to the National Oceanic and Atmospheric Administration.

A watch is either upgraded to a warning or canceled depending on the severity of the tsunami.

According to the weather service, "A Tsunami Warning means that all coastal residents in the warning area who are near the beach or in low-lying regions should move immediately inland to higher ground and away from all harbors and inlets including those sheltered directly from the sea. Those feeling the earth shake, seeing unusual wave action, or the water level rising or receding may have only a few minutes before the tsunami arrival and should move immediately. Homes and small buildings are not designed to withstand tsunami impacts. Do not stay in these structures."

Read more from the National Weather Service about conditions around the Bay Area here.

This story was written by Bay City News, with reporting contributed by Derek Wilson.

TSUNAMI SAFETY RULES FROM THE NATIONAL WEATHER SERVICE

• A strong earthquake felt in a low-lying coastal area is a natural warning of possible, immediate danger. Keep calm and quickly move to higher ground away from the coast.
• All large earthquakes do not cause tsunamis, but many do. If the quake is located near or directly under the ocean, the probability of a tsunami increases. When you hear that an earthquake has occurred in the ocean or coastline regions, prepare for a tsunami emergency.
• Tsunamis can occur at any time, day or night. They can travel up rivers and streams that lead to the ocean.
• A tsunami is not a single wave, but a series of waves. Stay out of danger until an "ALL CLEAR" is issued by a competent authority.
• Approaching tsunamis are sometimes heralded by noticeable rise or fall of coastal waters. This is nature's tsunami warning and should be heeded.
• Approaching large tsunamis are usually accompanied by a loud roar that sounds like a train or aircraft. If a tsunami arrives at night when you can not see the ocean, this is also nature's tsunami warning and should be heeded.
• A small tsunami at one beach can be a giant a few miles away. Do not let modest size of one make you lose respect for all.
• Sooner or later, tsunamis visit every coastline in the Pacific. All tsunamis - like hurricanes - are potentially dangerous even though they may not damage every coastline they strike.
• Never go down to the beach to watch for a tsunami! WHEN YOU CAN SEE THE WAVE YOU ARE TOO CLOSE TO ESCAPE. Tsunamis can move faster than a person can run!
• During a tsunami emergency, your local emergency management office, police, fire and other emergency organizations will try to save your life. Give them your fullest cooperation.
• Homes and other buildings located in low lying coastal areas are not safe. Do NOT stay in such buildings if there is a tsunami warning.
• The upper floors of high, multi-story, reinforced concrete hotels can provide refuge if there is no time to quickly move inland or to higher ground.
• If you are on a boat or ship and there is time, move your vessel to deeper water (at least 100 fathoms). If it is the case that there is concurrent severe weather, it may may safer to leave the boat at the pier and physically move to higher ground.
• Damaging wave activity and unpredictable currents can effect harbor conditions for a period of time after the tsunami's initial impact. Be sure conditions are safe before you return your boat or ship to the harbor.
• Stay tuned to your local radio, marine radio, NOAA Weather Radio, or television stations during a tsunami emergency - bulletins issued through your local emergency management office and National Weather Service offices can save your life.