There was a magnitude 3.8 aftershock 10 minutes after the initial quake, and there have been about 90 smaller shocks since.
Experts say the biggest risk of a large quake is in the first hours after the initial tremor. Experts say the quake was in the area of the Whittier Fault, an area that had a 5.9 shaker back in 1987.
There is a system of small faults in that Whittier region and officials are still trying to find the exact fault that ruptured.
"We need to have a well-located distribution of aftershocks because the aftershocks outline the section of the fault, the patch of the fault that broke during the main shock," said Caltech Seismologist Kate Hutton. "And once we see that, we'll be able to choose between the options that we have."
Many businesses throughout Southern California had merchandise and tools falling everywhere when the earthquake hit. Video caught many people flustered as the ground shook.
A few buildings in Pomona suffered damage. Mount San Antonio College held classes Wednesday, but some buildings remain closed.
"Most of our buildings, 40 percent of them, have what we're calling minor damage; where some ceiling tiles or light lenses have come down, or furniture has been moved about. But considering our campus is over 60 years old, we're in pretty good shape," said Jill Dolan, Mount San Antonio College.
Earthquake experts say this was not "The Big One," but it should be a wakeup call for those who are not prepared.
"I don't have water at home, I don't have food stored at home. No, I'm not prepared at all," said Pomona resident Tim Snyder.
Residents in Yorba Linda were up most of Tuesday night cleaning up after the earthquake.
Many people had cracked walls and stuck doors. One resident, Mingshu Lin, said just about everything came crashing onto the floor. Even the glass from a chandelier fell during the quake.
"All of a sudden the glass, like, fell on me and I ran," said Melanie Lin, Minghsu's daughter.
Melanie was not hurt in the quake, but Mingshu's water heater suffered damage.
"We found leaking water, so we had to have the repair man come by and fix the problem," said Mingshu.
Sales of hot-water-heater supplies and earthquake-preparedness items are up at local hardware stores.
Those at the hardware stores say it seems some people are getting prepared for the next earthquake.