Seismologists say the quake was centered 5 miles south of Ocotillo. It was an aftershock of the April Easter Sunday quake, which registered as 7.2 on the Richter scale causing serious damage. The April 4 quake was centered approximately 85 miles east of San Diego.
Seismologists are concerned about the general location of the quakes. Monday's quake's epicenter was approximately 50 miles north of the Easter day quake. It's part of the Elsinore fault line which connects to the Los Angeles basin.
"The Elsinore fault is capable of a major earthquake like a 7.5 or so, and it hasn't had an earthquake like that for at least 100 years," said Dr. Egill Hauksson of Caltech. "It could mean something and it could mean nothing."
Seismologists say they'll simply have to monitor the activity, which has been particularly busy lately. There have been more earthquakes in the last six months than the entire decade before that.
Monday's quake struck about 4.2 miles deep and lasted for at least three minutes, according to the United States Geological Survey.
"Probably people will be feeling earthquakes through the night occasionally," said Caltech Seismologist Kate Hutton. "Normally the largest aftershock of a quake, whether it's an aftershock or not, is about one point less than the main shock. So we might have a 4.7 some time. We don't know. It's all statistics."
"In California we always have to be prepared for 'The Big One' because we don't know when it's going to happen," said Hutton.
Eyewitness News received calls from viewers from all across the Southland, including Rancho Mirage, Yucaipa, San Jacinto, Yorba Linda, Tustin, East L.A., Anaheim, Inglewood and Long Beach reporting the earthquake.
The temblor forced a halt to the Toronto Blue Jays-San Diego Padres game in San Diego. A player had just grounded out in the eighth inning when Petco Stadium began to sway. Game goers saw the foul ball pole waving side to side. Everyone remained calm, and play resumed soon thereafter.
The quake has triggered more than approximately 400 aftershocks since 9:30 p.m. Monday. Seismologists say that strong jolts are not unusual quake such as the one in April.
"Well, it's sort of a misconception that aftershocks continue to get smaller. Actually, what they continue to do is get less frequent in time. And there still occasionally can be a big one later on," said Hutton.
The Los Angeles Fire Department went into earthquake mode, but there are no reports of injuries or serious damage.