Seismologists call them small to moderate earthquakes. They had enough force to be felt over a wide area of Southern California, but not enough intensity to cause much damage.
According to Caltech seismologist Kate Hutton, there were approximately 30 earthquakes since Tuesday night, but only three were big enough for residents to feel.
Hutton said there were two 4.5-magnitude earthquakes, one on Tuesday night and another on Wednesday morning, as well as a 3.4-magnitude quake. Seismologists say they were near the Whittier fault, but they believe they were associated with the Yorba Linda fault.
"This is all part of the same earthquake sequence. They're all in the same area," Hutton said.
Hutton said there are a few ways to look at the event. One is that there was a double earthquake, with two main shocks. The other way is to consider this an earthquake swarm, where there are a series of earthquakes that are about the same size or smaller.
"Either way, this is fairly common activity in California. We haven't had anything in the L.A. Basin the last few years, but that doesn't mean we're totally quiet," Hutton said.
There is a one in 20 chance in the first three days that the two earthquakes are foreshocks of another quake of the same size or larger. But as the hours pass, the likelihood of that happening diminishes.
According to the U.S. Geological Survey, Wednesday's 4.5-magnitude quake hit one mile north-northwest of Yorba Linda and four miles east-northeast of Placentia at 9:33 a.m. PT. Along with much of Orange County, residents in Whittier, West Covina, Long Beach, North Hollywood and San Bernardino reported feeling the quake.
The quakes in California are considered shallow, 8 to 10 miles underground. It appears these were strike-slip quakes: two tectonic plates touching as they slip past each other horizontally.
Mikeline Skibsted said she was shopping at the Wholesome Choice grocery store in Anaheim Hills at the time of the shaking.
"We were getting our produce for the day, and all of a sudden, it just shook, and we heard these jars coming off the shelves. After, we walked through and saw all the broken glass," Skibsted said. "It's kind of scary."
There were no reports of injuries or major damage. Seismologist Lucy Jones said the earthquakes provided a good demonstration of their earthquake early warning system. They have had a prototype of the system in operation since January. On Tuesday night, the system gave a 9-second warning before the 4.5-magnitude quake hit.
Hutton said the recent earthquakes are a good reminder to check earthquake supplies.
"Make sure you haven't borrowed anything out of your kit, that your food is up to date," she said. "Remember that we always have to be prepared in California."
NOTE: Tuesday night's earthquake was originally reported as a 4.4, but it was later upgraded to a 4.5.
