At least seven small earthquakes rattled Yorba Linda on Thursday and Friday, part of what appears to be a surge of seismic activity in the area in recent years, according to the U.S. Geological Survey.
No major damage or emergencies have been reported, said a spokesman for the Orange County Fire Authority.
The largest of the quakes, a magnitude 4 temblor with an epicenter about a mile north of the city, struck at 8:27 p.m. Thursday; the smallest registered 1.8.
A magnitude 3.1 quake hit at 4:06 p.m. Friday.
"It seems like that spot is relatively active, maybe a little more this last year," said Lucile Jones, a Geological Survey seismologist.
When Jones searched the historical catalog for earthquakes in a 150-square-mile area around Yorba Linda in the last 80 years, she found about 70 stronger than magnitude 3. About a third of those have struck in the last nine years.
Although it is difficult to tell which fault ruptures in earthquakes so small, Jones said the most recent Yorba Linda swarm appeared "consistent with motion on the Whittier fault." (The magnitude 5.9 Whittier Narrows earthquake in 1987 was actually not on the Whittier fault, but another, previously unknown fault nearby.)
A nearby part of the Whittier fault likely contributed to the magnitude 5.4 earthquake that struck Chino Hills last year, but the most recent Yorba Linda quakes appeared to rupture from a different, shallower location, Jones said.
The recent swarm also appears too far away to be related to the smaller swarm of quakes in the Chino Hills area in March.
Scientists have logged an increased number of quakes magnitude 3 and above in the last year, but they're not sure what it means.
"We have built this city on top of a web of faults," Jones said. "They're going to continue to be happening."