A pair of loud sonic booms heard around the Seattle area shortly before 2 p.m. Tuesday were caused by two F-15 jets, pursuing a seaplane that entered restricted airspace.

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Two loud sonic booms heard throughout the Seattle area Tuesday afternoon were caused by a pair of F-15 jets pursuing a seaplane that breached a 10-mile restricted zone around the president’s Air Force One.

The jets were sent off at 1:38 p.m. from the 142nd Oregon Air National Guard Wing in Portland, said spokeswoman Maj. Melinda Lapore. Fighter planes are on alert around the clock there to defend the Northwest coast from Canada to Northern California, she said.

At the time, President Obama’s plane was parked at Boeing Field during his tour of Seattle on Tuesday, which included a stop at an eatery, the Westin Hotel and one other fundraiser.

The seaplane landed in Lake Washington before the fighter jets arrived before 2 p.m., but after they “went supersonic near the Seattle area” minutes earlier, said the North American Aerospace Defense Command (NORAD) in Colorado.

The pilot of the seaplane was returning from Lake Chelan to Kenmore, via Stevens Pass, a route he regularly takes, said Todd Banks, president of Kenmore Air, which owns a floatplane facility at the north end of Lake Washington.

The air harbor is about a half-mile inside the 10-mile restricted area. Kenmore Air told its customers beforehand that it would be grounding its own planes for six hours.

Lesser restrictions applied for 30 miles around Boeing Field.

The aircraft was a Cessna 180, registered to pilot Lee Daily, of Kirkland, according to FAA records.

Banks said the pilot didn’t check the notices of air restrictions before leaving Lake Chelan on Tuesday morning.

The pilot and his passenger drove away from Kenmore after being interviewed by the Secret Service, and things were getting back to normal, Banks said, about two hours after the scare.

Passenger Laura Joseph, of Normandy Park, said she saw one of the two white jets off to the side. She and the pilot weren’t aware the president was in town, she said.

Only after Kenmore police met them on the dock did Joseph learn exactly what was going on, she told The Seattle Times.

Joseph told The Associated Press she didn’t hear the sonic booms.

“Oh my God, I can’t believe — is this the top news thing?” she said.

Indeed, the incident seemed to overshadow the president in online news dispatches, and by late afternoon, “Obooma” T-shirts were advertised online.

The FAA is investigating. Spokesman Allen Kenitzer wouldn’t speculate on what penalties the pilot might face until an investigation is done, perhaps weeks from now.

“We really don’t know what happened here,” he said. The inquiry should be relatively fast, he said, because it will be helped by radar data.

So many calls related to the sonic booms were made to the Pierce County 911 call center that it shut the operation down for about 15 minutes, according to detective Ed Troyer, spokesman for the Pierce County Sheriff’s Office.

In flight, an aircraft produces waves of air similar to those created in water by the bow and stern of a ship. When the aircraft exceeds the speed of sound (approximately 767 mph at sea level), the pressure waves are forced together or compressed into a shock waves.

The explosion-like sounds generated by these shock waves are called sonic booms.

Fighter jets respond to violations of restricted airspace “a couple hundred times a year,” said NORAD spokesman John Cornelio in Colorado.

A woman who works in Pioneer Square likened the “booms” to bombs going off and thought the explosions might be from a pirate ship left over from Seafair.

Duncan Turner, 57, of Lake Forest Park, was sitting in his downtown office when he heard the booms.

“It sounded far away like it could have been in Kitsap County,” he said. Turner almost shrugged it off until he got an e-mail from a friend asking if he had heard it, too.

“With the president in town, it adds a little more concern,” Turner said.

Paul Selig was at his Burien home enjoying his day off when he heard the booms. “It rattled the windows in my house,” he said. “I think everyone in Puget Sound is shook up.”

The booms showed up on seismographs, most dramatically at The Evergreen State College in Olympia, said Bill Steele, information-services director for the Pacific Northwest Seismology Network.

There were two distinct pulses, about 15 seconds apart, he said.

“Folks at the south end of the Sound seemed to get it more intensely than Seattle,” Steele said.

Seattle Times staff photographer Dean Rutz, reporters Susan Gilmore and Carly Flandro and news researcher Gene Balk contributed to this report.