A 6.6-magnitude earthquake centered on the Kona side of the Big Island — the largest to rattle the Islands in 23 years — and a 5.8-magnitude quake that followed disrupted air traffic on all major islands yesterday, sent boulders crashing onto Big Island highways and knocked out power throughout most of the state.

By 10 p.m., a slight majority of Hawaiian Electric Co. Inc.'s 291,000 customers — 157,000 — had power restored.

But Honolulu Board of Water Supply officials urged their customers to continue conserving water through this morning to ease the burden on the electricity-reliant system.

All of Maui's power customers had service restored by last night. Some 99 percent of the Big Island had service returned by 9 p.m., according to HECO, citing Big Island power officials. Kaua'i lost power temporarily, but service was fully restored last night, Kaua'i police said.

Until 6:15 p.m., Honolulu International Airport survived on emergency generator power.

Several flights out of Maui, the Big Island and O'ahu were disrupted or canceled. Thousands of passengers stood in long lines because some security checkpoints lacked power and Ho-nolulu International Airport's agricultural inspection system suddenly had to rely on dogs.

Electrically powered jetways could not be used in Honolulu, so passengers had to climb up stairs to board their flights.

Although visitors and residents on all islands were shaken or awakened by the 7:07 a.m. temblor and the second quake, there were no fatalities.

The most serious injury was a broken arm on the Big Island, Gov. Linda Lingle said. Big Island officials said a survey of emergency rooms showed 25 people suffered minor injuries.

This morning, several Big Island public, private and charter schools will be closed. State Department of Education officials said public school teachers and administrators have to report for inspections and cleanup, although students are off.

No other islands' public schools are scheduled to be closed. University of Hawai'i at Manoa spokesman Jim Manke said classes would be held today if electricity was up.

STEADY SHUDDER

The first quake hit at 7:07 a.m. and was felt as a steady, rumbling shudder that shook houses, cracked buildings, swayed high-rises and knocked artwork from their walls and sent countless chachkas crashing to the ground.

It was located 24 miles below Kiholo Bay on the Big Island, according to the U.S. Geological Survey Hawaiian Volcano Observatory, and was triggered by tectonic movement, not volcanic activity.

A second quake, of 5.8-magnitude, hit just seven minutes later. It was located 13 miles northwest of Kawaihae.

Beginning at 10:30 a.m., a series of 55 aftershocks followed, said Jim Kauaihikaua, scientist in charge of the U.S. Geological Survey Hawaiian Volcano Observatory.

The largest aftershock struck at 10:35 a.m. and had a magnitude of 4.2. It was located west-northwest of Kawaihae, Kauaihikaua said.

Despite all of the seismic activity, there was no threat of a tsunami.

Sarah Lee of Kailua, Kona, was at church at 7:07 a.m. when "everything started shaking and the road outside was rolling a bit," she wrote to honoluluadvertiser.com.

Driving home, Lee saw "plenty of knocked-over rock walls as well as an old coffee shack that was tipped over on the side of the hill."

Jacob Ellis of Mililani woke yesterday to find everything shaking.

"My mom had taken out a bunch of food and started throwing it all on the grill and we all ended up having a really interesting breakfast," Ellis wrote. "I've never seen anything like it before. People who were driving were really careful and let everyone have their turn. It was nice seeing everyone in our community showing so much aloha."

A QUIET SUNDAY

Ke'aka'okalani J. Garay, a CT scan technologist, was working at The Queen's Medical Center when he felt a "completely odd" rumble that quickly caused the department to shake "vigorously and uncontrollable."

"I called my dad in Texas to tell him I loved him," Garay said. "We now sit here and pray that our services will not be needed."

Hundreds of Hawai'i families experienced something extraordinarily unusual yesterday: a quiet Sunday with conversation, board games and no television or Internet use.

Waynette Tsubota of Kahalu'u and her family usually spend their fall Sundays watching football.

Instead, three generations had an impromptu barbecue yesterday in the carport, watching cars drive by on Kamehameha Highway.

Tsubota considered the day a blessing.

"We got to spend time together in the garage with no TV distractions, with the family sharing things," Tsubota said. "This is the closest we've been in a long time."

The quake hit just a month after a military truck crashed into the 'Aiea Overpass of the H-1 Freeway, turning nearly the entire island of O'ahu into rush-hour gridlock for hours.

CAUGHT UNPREPARED

Yesterday's event also shook thousands of Hawai'i residents into the realization that they had not prepared for a major earthquake and could not properly feed their families without electricity.

Just moments after the quake, hundreds of customers began lining up outside O'ahu supermarkets waiting for workers to escort them inside by the light of only a flashlight.

Some stores, such as the Safeway at Aikahi Park Shopping Center, accepted only cash and rang up purchases with paper and pen.

Stores shelves on all major islands were quickly emptied of items such as ice and batteries.

Yesterday's quakes knocked out power on parts of the Big Island, Maui and all of O'ahu.

Unlike the older, diesel-based generators on the Neighbor Islands, O'ahu's more modern, complicated turbine-driven system took much longer to come back on line, leaving drivers to navigate busy intersections without traffic signals or street lights.

O'ahu's power grid failed when the earthquakes knocked out generators at HECO's Downtown and Kahe power plants.

The downed generators triggered an imbalance in the system, prompting HECO's computer system to shut down the entire system to avoid permanent damage and cause a more prolonged power failure, HECO officials said.

After HECO restarted its Waiau power plant, residents and businesses regained service in parts of Pearl City, 'Aiea, Waikele, Waipahu, Mililani Mauka, the Honolulu Airport area, Makalapa, Kunia, Wai'anae and Nanakuli.

Just over an hour after the first quake hit, half of the Honolulu Fire Department's nearly 50 companies were out on power-outage calls by 8:30 a.m.

Most of the calls were for people stuck in elevators, said Fire Capt. Frank Johnson.

NO ESTIMATED DAMAGE

Federal and state officials had no damage assessment yesterday and will send out teams today, Lingle said.

State officials will seek an emergency declaration that would lead to federal reimbursement of state costs, as well as a major disaster declaration to cover major repairs.

Before yesterday, the last large quake to strike the Islands was the 6.7-magnitude Ka'oiki quake that hit the Big Island on Nov. 16, 1983.

O'ahu's rain-slicked roadways were unusually light as people heeded warnings by city and state officials to stay home. Some drivers obediently treated blank traffic signals as four-way stops, while others barreled through or skidded to stops.

Honolulu Police Chief Boisse Correa said police doubled the manpower at all major intersections to direct traffic on O'ahu, and more officers were brought in last night.

At least one accident — at Kapi'olani Boulevard and Ward Avenue — was blamed on the absence of a working traffic signal, according to the Honolulu Police Department.

While drivers, for the most part, stayed off O'ahu's roads and highways, thousands of people ignored pleas from government and telephone officials and could not stay away from their cell phones.

Their cell calls — and KSSK Radio — provided the main communication links for most of the day.

The Big Island took the brunt of the structural damage, and dozens of patients had to be evacuated from Kona Community Hospital. The hospital expects to be out of commission for two days.

Lingle was in her hotel room at the Mauna Lani Resort yesterday when the quake hit.

Lingle later surveyed the Big Island in a Drug Enforcement Agency six-passenger helicopter from which she saw rocks and earth falling into Kealakekua Bay near the Captain Cook monument and witnessed the evacuation of Kona Community Hospital.

On O'ahu, the loss of electricity caused an estimated 15,500 gallons of partially treated sewage to spill into Lake Wilson when the ultraviolet disinfecting unit at the Wahiawa Wastewater Treatment Plant lost power, according to the city's Department of Environmental Services.

Before emergency generators could kick in at the Sand Island Wastewater Treatment Plant, a "clarifier" overflowed and spilled an estimated 1,200 gallons of partially treated wastewater within the plant.

Reach Dan Nakaso at dnakaso@honoluluadvertiser.com.