Fault that triggered 7.0 earthquake off California not known to produce sizable tsunamis

The fault zone responsible for Thursday’s 7.0 magnitude earthquake off the coast of California is not known to produce large tsunamis, but due to the size of the event, NOAA’s Tsunami Warning Center issued alerts for more than 4 million residents along the West Coast of the U.S.

The Mendocino Fault Zone, the region responsible for the quake, is home to strike-slip faults, which involve horizontal motion rather than vertical displacement that would typically move larger amounts of water. The United States Geological Survey says there have been at least 40 other earthquakes of a magnitude 6.0 or larger in the region during the last 100 years.

The 7.0-magnitude earthquake did produce a small tsunami, but it measured less than two inches in height, according to initial data.

The small rise in water did not cause any reported damage and was significantly smaller than the tsunami generated by the 2022 Tonga volcanic eruption, which saw several feet of water impacting parts of the California coastline, ripping boats from their moorings and damaging docks.

In contrast, the Cascadia Subduction Zone, which extends from northern California to southern British Columbia, has a greater potential to produce a large tsunami, experts warn.

This is because a tectonic plate is forced under another, displacing massive amounts of water and triggering a potential powerful tsunami during the vertical motion. 

A 9.0 quake in 1700 in the subduction zone sent a devastating tsunami along the West Coast and even across the Pacific to Japan.

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Why were tsunami warnings issued Thursday? 

So why were warnings issued for the 7.0 earthquake, even though a sizable tsunami was unlikely?

Seismologists explained that the decision to issue warnings was based on the data available at the time.

Thursday’s quake was significant and occurred close to the coastline, leaving little room for error and scant time to react.

At the time of the warning’s issuance, there was no data to confirm whether a tsunami was on its way, so alerts remained in effect until observations determined there was not a larger threat.

Past events, such as the 9.0 earthquake in Japan in 2011 and the 2022 Tonga eruption, were thousands of miles away, providing valuable time for officials to assess the situation and issue the necessary alerts to warn those along the West Coast about potential water rise.

Had there been a sizable tsunami during Thursday’s event, it would have reached the coastline in just minutes, unlike larger events in the Pacific Basin, which typically take 8 to 12 hours to reach the coast.

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Some seismologists considered the event as a good test to help discover shortfalls in residents’ preparations and responses to a major event.

Some websites that house important information regarding tsunamis and evacuation zones were down in the immediate aftermath of the 7.0 earthquake – a likely casualty from the flood of people seeking advice.

California Gov. Gavin Newsom issued a state of emergency for three Northern California counties but only isolated damage was reported as of Thursday evening.

“Today’s emergency proclamation will allow more resources to go where needed for emergency response to this morning’s earthquake,” Newsom said in a statement. “I am grateful for the robust system our state has in place that worked as intended today and kept people safe and informed.”