Striking in the early morning a few days after the largest earthquake drill in U.S. history, Monday's magnitude 4.1 shaker was a wake-up call for residents of San Diego and Riverside counties, but little more.

After jolting people out of their beds as far away as Twentynine Palms and the U.S.-Mexico border, the earthquake centered 10 miles north of the Palomar Observatory was followed by a tiny aftershock of magnitude 1.0 five minutes later.

That was followed by a much larger aftershock of magnitude 3.8 at 9:41 a.m., according to the U.S. Geological Survey.

All three quakes came from roughly the same location just north of the Riverside-San Diego county line, near the community of Aguanga. The shaking was centered 20 miles east of Temecula and 30 miles north of Escondido.

No injuries or damage were reported.

While some people felt all three shakers, the epicenter of the talk was on the strongest initial jerk.

"We felt the earthquake pretty well, but nothing fell off shelves," said Pam Nelson, who lives near the county line. "It was just a big jerk."

Ida Martin of Aguanga said it was a strong enough jerk to wake her up.

"Nothing fell. No pictures collapsed. Everything was fine," Martin said.

But it shook her two cats, who were on the bed at the time.

"The cats felt it because they looked right at me, as if to say, 'What was that?'" Martin said.

Battalion Chief Cliff Kellogg of the Palomar Mountain Volunteer Fire Department said he was already awake when the first quake hit.

"I felt kind of a wavy little movement and all of a sudden she hit," Kellogg said.

Kellogg said he and Chief George Lucia immediately went to the fire station to temporarily move vehicles out of their bays and assess whether the station or the engines had been damaged. They had not.

As scientists gathered to assess the implications of the flurry of quakes, they were intrigued by the size of the second aftershock.

Debi Kilb, a seismologist with UC San Diego's Scripps Institution of Oceanography, said it is quite rare to have one that strong. Normally, she said, aftershocks tend to be at least one magnitude below the initial quake.

Kilb also said seismologists will be watching to see if there is any other shaking over the next few days. There is a small chance, though an unlikely one, that the two jolts of comparable size could signal that a swarm of similar-sized quakes are set to hit, she said.

Kilb said she, too, felt the jolt in her two-story home in Carmel Valley.

"It was pretty short," she said. "It was a strong burst and then it ended."

Contact staff writer Colleen Mensching at (760) 739-6675 or cmensching@nctimes.com. Contact staff writer Dave Downey at (760) 745-6611, Ext. 2623, or ddowney@nctimes.com.