A “non-seismic event” triggered a report of a 4.0 magnitude earthquake Friday morning off the coast of Southern California, the USGS said.

The earthquake report released before 10 a.m. said the quake was centered in the ocean about 18 miles southwest of Avalon on Catalina Island, according to the US Geological Survey. People have also reported shaking on the USGS Did You Feel It page.

The USGS then tweeted that the earthquake detection system had been triggered by something else.

“ShakeAlert detected an event other than an earthquake that triggered our system,” the USGS said in a tweet. “Fortunately, no public alerts were sent. This is a bit of a first for us as these events happen so rarely. Apologies for any confusion in our previous tweets. The #ShakeAlert team is now making improvements in based on what we learned from that one event.”

Details on what triggered the system were not immediately available. Human activities, such as mining and construction explosions, can sometimes create seismic waves large enough to be detected by the USGS National Seismic Network.

The reported earthquake was later removed from the USGS Map of Recent Earthquakes and Did You Feel It? page, where people reported mild shaking in Long Beach, Irvine and other places.

Santa Catalina Island, one of the Channel Islands off the California coast, is located southwest of Los Angeles. The towns of Two Harbors and Avalon are located on the island.