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Alaska earthquake triggers SF tsunami watch

Although danger has passed, city warns people to keep away from the beaches

San Franciscans woke up to an emergency text warning of the possibility of a tsunami early Tuesday morning, the result of a magnitude 7.9 earthquake in Alaska hours prior.

Although the danger of a catastrophic wave subsided shortly, the city’s Department of Emergency Management still says to remain vigilant for the time being.

The U.S. Geological Survey [USGS] recorded the quake off the Alaskan coast at 1:32 a.m. Pacific Time. Although the tremor was large, USGS has projected only light damage so far.

The National Tsunami Warning Center in Palmer, Alaska issued a tsunami watch for the entire West Coast of the United States, including San Francisco. But a few hours later the center announced that it had cancelled the watch everywhere except for areas adjacent to the quake itself.

Even though the coast is clear, the San Francisco Department of Emergency Management warned via Twitter that San Franciscans should keep away from coastal areas today.

Note that the hypothetical inundation zone, where monster waves could smash against the waterfront, extends not just along the length of Ocean Beach but also on the bay itself.

Indeed, the danger of an unstable wave hitting the east side of the city after a tsunami pushes huge amounts of water through the bay—called a seiche wave—is probably worse than the danger from the tsunami itself.

The National Oceanic and Atmospheric Association explains the phenomena:

A seiche may occur in any semi- or fully-enclosed body of water. Seiches are typically caused when strong winds and rapid changes in atmospheric pressure push water from one end of a body of water to the other. When the wind stops, the water rebounds to the other side of the enclosed area.

The water then continues to oscillate back and forth for hours or even days. In a similar fashion, earthquakes, tsunamis, or severe storm fronts may also cause seiches along ocean shelves and ocean harbors.

There has never been a large and dangerous tsunami in the Bay Area in recorded history, although a tsunami did technically reach the city in 2011.

The 23-foot wave originating from a 9.0 quake in Japan was only a foot high by the time it reached our shores, which was still enough to cause some property damage.

In 2015, Stanford seismologist Greg Beroza told San Francisco Magazine that the local San Andreas fault could in theory generate a tsunami off the coast of California (albeit only incidentally, by triggering an underwater landslide or volcanic eruption of sufficient size), but such a wave would be “a roiling mess” rather than the perfectly formed crest we usually associate with the word.

USGS warns those living in tsunami zones—which, as a reminder, means San Francisco too—to heed official warnings even if they seem ambiguous or if there seems to be no visible danger.

A tsunami may consist of multiple waves, so don’t assume the danger has passed once one subsides. In the event of a tsunami, move away from the coast and take shelter in the highest accessible area.