San Diego, California - A pair of small earthquakes occurred one second apart late Sunday on different fault systems south of the US-Mexico border, producing light shaking in areas of San Diego County, according to the US Geological Survey.

"That's absolutely crazy," said Tom Rockwell, a seismologist at San Diego State University. "It's very random that quakes on separate faults happen at about the same time."

The first quake was a 3.4-magnitude temblor that hit at 5:13 PM PT about 5 miles west-southwest of Tecate, Mexico, and 15 miles west-southwest of Campo. It was immediately followed by a 3.6 quake about 23 miles west-southwest of Progreso, Mexico, and 31 miles east-southeast of Campo.

There was no immediate indication that shaking on the first quake triggered the second.

Small quakes regularly occur in and around the US-Mexico border, but rarely lead to large events, Rockwell said.