A powerful earthquake Monday shook fishing villages along Mexico's Gulf of California and prompted alarm as far away as Phoenix, but there were no immediate reports of injuries or damage.

The U.S. National Earthquake Information Center said the 6.9-magnitude quake struck at 12:59 p.m. (1:59 EDT, 17:59 GMT) and was centered 76 miles north-northeast of Santa Isabel in Baja California and 331 miles southeast of the border city of Tijuana.

It was the strongest of four quakes of 5.0-magnitude or greater that struck the area over a 45-minute period late Monday morning.

Phoenix Fire Department spokesman Alex Rangel said a high-rise near downtown shook violently enough that workers evacuated, but there were no reports of injuries or damage. The quake was centered about 460 miles from Phoenix.

Wilfredo Rivera, a manager at the Posada Santa Gemma hotel in Bahia Kino near the coast, said doors slammed as the ground rocked.

"The earth was turning around really ugly," he said. "People got really scared."

The quake was also felt in San Diego, where city employees left an 18-story downtown tower that houses the city attorney and other departments.

"Employees heard and felt some shaking and rattling," said Darren Pudgil, a spokesman for Mayor Jerry Sanders. "I'm told 40 to 50 people left the building but have since returned."

Gina Coburn, a spokeswoman for the city attorney's office, sought safety in a doorway of her 16th-floor office and thought briefly about leaving the building.

"I felt it a lot," she said. "The building was swaying back and forth. I didn't know if I should go down the stairs."

Civil protection officials in the two Mexican states on either side of the quake — Baja California and Sonora — said there were no reports of damage or injury.

The quakes were all centered in the middle of the narrow slice of sea between the Baja peninsula and Mexico's mainland, which reduced its chances of causing major damage, said Don Blakeman, an analyst at the earthquake center.

Earthquakes around 7.0 magnitude are common in the region, according to the center, including a 7.6-magnitude quake that hit the coast of Colima state in 2003.

Seismologist Debi Kilb at Scripps Institution of Oceanography in San Diego said the series of quakes started with a 5.8 preshock followed quickly by a 6.9 temblor. She said two aftershocks came about half an hour later.

Kilb said it wasn't surprising that the quakes didn't cause damage. They were in the water, far from major cities, she said.

U.S. authorities said there was no tsunami threat to Hawaii or the Pacific coast of the United States. The Gulf of California coast was put on alert for large waves, said Alfredo Escobedo, the director of the Baja California civil protection service.

Scientists say some areas where strong shaking occurred may experience local underwater landslides.

The Gulf of California, also known as the Sea of Cortez, is believed to have come into being millions of years ago when tectonic forces shifted the Baja California peninsula off the North American Plate.

Called the "world's aquarium" by undersea explorer Jacques Cousteau, the sea has a unique ecosystem and is home to hammerhead sharks, whale sharks, orca whales, dolphins and giant Pacific manta rays.

Click here for more information from the U.S. Geological Survey.