Thursday morning the National Weather Service (NWS) issued a tsunami warning from Oregon to the Bay Area. The warning, which was later canceled, followed a 7.0 magnitude earthquake that occurred off the coast of Northern California at 10:44 a.m., according to the Associated Press.
This was one of many earthquakes to shake California this year, but one of few to prompt a tsunami warning.
“Earthquakes cause all sorts of what we call secondary hazards. They can cause fires. They can cause dust storms and they can cause disease outbreaks by shaking dust that’s full of germs,” said Sylvain Barbot, an associate professor of earth sciences at USC. “When an earthquake is offshore, there’s inevitably some tsunami.”
Phones in Northern California received warnings from the NWS and local agencies, urging people near the coast to avoid low-lying areas and to move to higher ground immediately. Local fire departments assisting and assessing the damages of the earthquake were forced to shift focus following the warning.
“We start working with our other public safety agencies in the neighborhood. Pretty much any vehicle with a loudspeaker public address system is going to go around and start getting people away from the water and off of the coastlines,” said Justin Shorr, a rescue captain of the San Francisco Fire Department. “We’re trying to get them towards predetermined tsunami evacuation routes, which are signposts along the streets that can lead folks away from the water and to an area of high refuge.”
Shorr said that the warning issued on Thursday was the highest level of warning issued to give communities ample notice to evacuate.
“This was not a false alarm. This was not a mistake. The system worked exactly how it was supposed to. We just got, I don’t want to say we got lucky this time, but what a great chance to see if we are as prepared as we think we are,” Shorr said.
Despite frequently being affected by earthquakes, California surprisingly does not face as much tsunami danger. Many earthquakes felt in California occur inland, not causing any tidal change even if the magnitude is high.
Earlier this year, a 4.7 magnitude earthquake shook Malibu near the coast but did not prompt the same tsunami warning as the one in Northern California on Thursday. Barbot said the type of tectonic plate and the direction they move in plays a significant role in whether an earthquake causes a tsunami or not.
“The one in Malibu is on what we call the thrust faults, and they are really when one pile of rocks climbs above another. And so when it does that, some rocks go up and some rocks go down. And so that creates vertical motion. When that’s offshore, it can be accompanied by a large tsunami,” Barbot said.
Since the Malibu earthquake shook land, there was no tsunami warning. Thursday’s earthquake shook off the coast near Eureka, but since the tectonic movement was side to side, it was less likely to cause a tsunami.
Costas Synolakis, founder of the USC Tsunami Research Center, said that under certain conditions, even side-to-side earthquakes can create tsunamis, hence why the NWS issued an early warning.
“When the earthquake is very close to the coast, there is no time immediately afterwards to know exactly what the earthquake motion was. So typically, we are on the side of caution, and we issue an alert right away, so that agencies can start the evacuation process,” Synolakis said.