(Updates with magnitude downgraded to 6.9, quote from president, suspension at oil refinery)
By Rosalba O'Brien
SANTIAGO, April 24 (Reuters) - A strong earthquake of magnitude 6.9 struck off the west coast of Chile on Monday, rocking the capital Santiago and briefly causing alarm along the Pacific Coast but sparing the quake-prone nation of any serious damage.
The quake was centered about 85 miles (137 km) from Santiago, and some 22 miles (35 km) west of the coastal city of Valparaiso. The U.S. Geological Survey twice revised the magnitude before settling on 6.9, a strength usually capable of causing severe damage.
The epicenter's shallow depth of 15.5 miles (25 km) below the sea allowed it to be felt hundreds of miles (km) away. Santiago office buildings swayed for about 30 seconds at the end of the workday.
Closer to the epicenter, residents scrambled for higher ground, remembering the lessons of the country's devastating earthquake and tsunami in 2010.
"It was short but very powerful," said Paloma Salamo, a 26-year-old nurse, who was in a clinic in Viña del Mar, just north of Valparaiso, when the quake struck.
People ran from the facility carrying children and some headed for the hills when the tsunami alarm sounded, she said, but calm was soon restored.
"So far there has been no human loss nor significant damage," President Michelle Bachelet said, praising people for evacuating in an orderly fashion in the immediate aftermath.
Officials canceled a tsunami warning that had been issued in Valparaiso. The Pacific Tsunami Warning Center reported small tsunami waves of half a foot (15 cm).
There were no reports of structural damage in Valparaiso, but cellphone networks were down in some places, a spokesman with the local government said.
Videos from the Valparaiso area showed objects falling from store shelves, rocks falling onto roads and lights flickering. The quake was felt as far away as Argentina, on the other side of the Andes.
Interior Minister Mario Fernandez said there had been some landslides but "in general the situation is pretty normal bearing in mind the quake's intensity."
Strict construction codes in Chile limit damage to buildings.
Copper mining was unaffected, according to Chile's state-run Codelco, one of the largest copper mining companies in the world, and Anglo American, which has copper operations in central Chile.
But interruptions in the electricity supply led the Aconcagua oil refinery to temporarily suspend operations for safety reasons, state-run oil firm ENAP said. There was no damage to either of Chile's two refineries, ENAP said.
Several aftershocks including two of magnitudes 5.0 and 5.4 were recorded in the same spot and could be felt in Santiago, part of a cluster of tremors from that area in recent days.
Chile, located on the so-called "Pacific Ring of Fire," has a long history of deadly quakes, including a 8.8 magnitude quake in 2010 off the south-central coast, which also triggered a tsunami that devastated coastal towns. More than 500 people died.
That was the sixth-largest earthquake ever recorded, according to the USGS. The largest recorded temblor in history was also in Chile, a 9.5-magnitude quake in 1960.
The long, slender country runs along the border of two tectonic plates, with the Nazca Plate beneath the South Pacific Ocean pushing into the South America Plate, a phenomenon that also formed the Andes Mountains. (Reporting by Rosalba O'Brien, Fabian Cambero, Gram Slattery, Felipe Iturrieta and Jorge Otaola; additional reporting by Sandra Maler in Washington; Writing by Daniel Trotta; Editing by G Crosse and Mary Milliken)