SAN DIEGO — After an underwater volcano erupted near the Pacific nation of Tonga, a tsunami advisory was issued for San Diegans Saturday, alerting them not to go into the water and to stay away from the shore.

“You get an explosion way down in the Southern Hemisphere like that, it’s definitely going to come on up here, the question is how much do we get,” said Dr. Pat Abbott, a geology professor at San Diego State University.

The advisory issued by the National Tsunami Warning Center advised residents waves would start to crash around 7:50 a.m., possibly reaching one to three feet. Dr. Abbott says the height of a tsunami is not the dangerous part, but the duration of the waves.

“The most dangerous part of it is not the height, it’s the period, the fact that it keeps coming and coming and doesn’t quit,” Dr. Abbott said.

According to the SDSU professor, San Diego is protected by natural barriers.

“When you look offshore and you see the Coronado islands and San Clemente islands and Catalina island, all those islands out there are barriers,” he said. “They are things that the tsunami has to crash against to get through to reach us.”

The advisory was canceled in the afternoon around 1:30 p.m. by the National Weather Service, prompting San Diegans to head back into the water.

“When we were driving out here, I’m like not too sure about this, but being out in the water, this is actually very fine,” said Alexandra Van Aleveek, who went surfing with her sister after the advisory was lifted. “We saw some dolphins, we saw some seals, so it was great.”

Dr. Abbott says the Hunga Tonga Hunga Ha’apai volcano is an active volcano — which erupted multiple times in 2014 and 2015 — creating small islands in the area that he believes is happening now.