A minor earthquake of 3.6 magnitude shook South Whidbey at 5:47 p.m. Thursday, but no injuries or damages were reported immediately following the temblor.

The quake, 20 miles deep, was centered in Clinton. Though originally reported at 3.3 magnitude, 18 miles below Puget Sound, seismologists at the University of Washington upgraded the trembler on Friday morning to 3.6.

The quake was briefly felt by South Whidbey residents, and was centered on Cultus Bay Road, about midway between Glendale and French Roads. The shake shook a sparsely populated, wooded portion of the south end of Whidbey Island, about 7 to 8 miles south of Highway 525.

Although it was centered near Clinton, many residents in that area were not aware of the earthquake, including Lynae Slinden of Clinton.

“It was right under my house, but I didn’t feel a thing. But my neighbors, Bob and Liz Arnold reported they felt it,” Slinden said.

One resident felt it as far north as Greenbank.

“We felt it shake our home on Classic Road,” said Tanya Foster who works at JW Desserts in Clinton.

John Auburn, owner of J W Desserts at Ken’s Korner, didn’t feel a thing.

“We working in the kitchen at that time and were not aware anything was happening,” he said.

Troy Cook was working in La Paz Restaurant in Clinton, and said people there didn’t notice the earth move, either.

“But customers came in later and told us about it,” Cook said.

Valerie Cohn from the Clinton Food Mart was also unaware of the quake.

“I was working, too. Nothing shook, we didn’t feel a thing,” Cohn said.

The Island County Sheriff’s Office did not receive any calls from worried residents.

“Residents did not contact our regional emergency dispatch center about the incident. Sheriff’s deputies on routine patrol in the South Whidbey area reported that the quake was of a very short duration,” said Jan Smith, spokeswoman for the Island County Sheriff’s Office. “At this time, there have been no reports of injuries, structural damage or trees down.”

The quake occurred deep inside the earth, University of Washington experts said.

“The computer picked up a deep crustal earthquake, then a smaller aftershock at 6:13 p.m.,” University of Washington seismologist Jon Connolly reported.

“A lot of people felt it because it was so deep,” he added. “When you put a flashlight against a wall the light is intense, but the light diffuses the farther away you are. Same thing with quakes.”

Eartquake measurements are done on a Richter scale, and based on a logarithmic calculation. What this means is that for each whole number that’s higher on the Richter scale, the amplitude of the ground motion recorded by a seismograph rises 10 times.

Using this scale, a magnitude 5 earthquake would result in 10 times the level of ground shaking as a magnitude 4 earthquake (and 32 times as much energy would be released).

The seismology lab at the University of Washington records roughly 1,000 earthquakes annually in Washington and Oregon. Between one and two dozen of these cause enough ground shaking to be felt by residents.

Most are in the Puget Sound region, and few cause any damage.