An earthquake that rocked the state at 9:20 a.m. yesterday was an aftershock of two Oct. 15 quakes that caused tens of millions of damage across the state, scientists said.

No tsunami was generated, but the shudder was felt on all islands, and Hawai'i County Civil Defense officials said they were watching for possible rockslides and minor damage. There were isolated power outages on the Kona side of the island.

"It was a deep event. It was felt widely because seismic waves propagate efficiently at depth. If it had been more shallow, it might have been felt only on the Big Island," said Chip McCreery, director of the Pacific Tsunami Warning Center.

The origin of the statewide shudder was almost precisely at the same coordinates as the October quakes, which had magnitudes of 6.7 and 6.0.

"It's most likely an aftershock of last month's event," McCreery said.

He said that although there have been numerous small earthquakes at the same location in the weeks since Oct. 15, this was the largest since that date.

The Pacific Tsunami Warning Center initially estimated the quake strength at magnitude 4.5, but within a few minutes upgraded it to 5.0. That number is still preliminary and may be further adjusted after a thorough review of the records of the event, a federal geophysicist said.

The rumble felt around the state wasn't as pronounced as that from the October quakes.

Motoo Shinomae, owner of Sunshine Scuba, said he barely felt the tremor yesterday morning on O'ahu.

"I didn't know it was an earthquake," he said.

Hawai'i County's emergency operations center was opened to handle Big Island issues. Civil Defense chief Troy Kindred said some radio stations that normally would be broadcasting emergency information were off the air due to power outages.

"That's something we're still working on," he said.

"I anticipate that we're going to see some slides from rocks that may have been dislodged Oct. 15 but didn't make it down the slope," Kindred said.

The Hawai'i Belt Road — Highway 19 — was closed at Laupahoehoe Gulch after a small slide blocked the way, said Scott Ishikawa, state Department of Transportation spokesman. A DOT crew from Hilo cleared small boulders and debris from the road and it was reopened by 11 a.m., less than two hours after it was closed, Ishikawa said.

A crew drove around the Big Island to check for other problems in other areas and at the piers of Kawaihae, he said.

"There was no additional damage there because of today's quake," he said.

The quake knocked out electrical service to about 5,900 customers in North Kona and Kalaoa on the Big Island for a little more than an hour, Hawaii Electric Light Co. said.

A U.S. Geological Survey Web site with information on the quake and previous shakers can be found at http://pasadena.wr.usgs.gov/shake/hi/.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com.