GUANICA, PUERTO RICO (NYTIMES, AP) - A 5.8-magnitude earthquake shook south-western Puerto Rico before sunrise Monday (Jan 6), frightening people out of their beds, cracking house walls and destroying a photogenic beachside rock formation known as Punta Ventana.

The natural wonder in the town of Guayanilla, shaped like a round stone window with a stunning view of the ocean, had begun to look vulnerable after smaller temblors started to hit the area a week ago, Mayor Nelson Torres Yordán said.

On Monday, he said, "it finally fell."

The quake struck at 6.32am local time, according to the US Geological Survey. It was the strongest yet to be felt in the coastal towns west of the city of Ponce that have been trembling for more than a week.

The rash of smaller temblors began with three shakes of 4.7, 5.0 and 4.7 magnitude in the space of three hours during the night of Dec 28-29 and have continued since then, clustered in the same area a few miles offshore.

No tsunami threat has been reported.

"It started shaking a bit, but then, all of a sudden, we felt a jolt - I'd never seen anything like it," said José Francisco Benítez, 48, who was awakened by the quake at a beach resort in the town of Guánica.

A strong, 4.9-magnitude aftershock struck about four hours after the big quake, rattling nerves again. Officials warned of possible mudslides and urged people to stay off the roads in the area to allow emergency personnel to assess the damage.

Monday is Three Kings Day, a holiday in Puerto Rico also known as the Feast of the Epiphany.

Mayor Santos Seda of Guánica, where some of the most serious damage was reported, told NotiCentro, a local news station, that at least four houses had collapsed. His interview was cut short when his telephone call to the station was dropped.

The Puerto Rico Electric Power Authority reported local outages as a result of the temblor. Photos posted on social media showed severe cracks in the walls of older raised homes in the area and partial collapses of some.

"People can expect to feel more earthquakes over the next few days, especially given its location near the coast," said Elizabeth Vanacore, a seismologist with the Puerto Rico Seismic Network.

One of the largest and most damaging earthquakes to hit Puerto Rico occurred in October 1918, when a 7.3-magnitude quake struck near the island's northwest coast, unleashing a tsunami and killing 116 people.