With his surfboard tucked under his arm, Alex Felton was about to walk a couple of blocks to San Francisco’s Ocean Beach on Thursday morning when his cellphone blared. “TSUNAMI WARNING!” an alert screamed. “You are in danger.”

A lifelong surfer, Mr. Felton, 31, has ridden 10-foot waves at Ocean Beach many times. He considered jumping into the ocean anyway, he acknowledged later. But texts from friends convinced him to follow the alert’s directions and move away from the coast, not toward it.

Which friends were worried that he might make a bad decision? “All of them,” he said with a grin.

Bay Area residents rode a metaphorical wave together on Thursday, after a 7.0-magnitude earthquake off the California coast shook the ground for hundreds of miles and prompted the National Weather Service to issue a tsunami warning for coastal counties in Northern California and southern Oregon.

First, blaring phone alerts scared the wits out of people across the region, their hearts pounding as they wondered what sort of disaster-movie-type scenario was about to occur. Then they calmed down and even laughed as the alert was called off about an hour later.