The fences will move in at Safeco Field in 2013, with the largest change from left-center field to center, where the fences will be as much as 17 feet closer to home plate.

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Mariners catcher Jesus Montero bore a smile as wide as any he’s displayed his rookie season.

He had just learned Tuesday about the team’s plans to move Safeco Field fences closer to home plate for the 2013 campaign. The biggest changes will be in left-center field, with the fence being moved anywhere from 4 to 17 feet and sure to benefit right-handed hitters like Montero.

“Right now, this is the biggest park in the league, it’s crazy,” Montero said. “So, that’s good news for us and everybody. Everybody cheers for the hitters, so hopefully, we’ll have fans rooting for us even more.”

Montero said the ballpark’s dimensions — unchanged since opening in 1999 — have been an open source of clubhouse frustration.

“I’m not going to lie,” Montero said. “Everybody knows that sometimes you crush a ball and you think that it’s gone and it’s not. Sometimes, you hit the ball hard and just hit the wall. So, it’s going to be awesome for us next year, for the hitters.”

The current fences didn’t keep Kyle Seager from depositing his 20th home run in a 6-1 victory over the Los Angeles Angels on Tuesday night in front of 14,353 fans. Seager became the first Mariner since Russell Branyan and Jose Lopez in 2009 to enjoy a 20-homer season.

Mariners president Chuck Armstrong and general manager Jack Zduriencik had assistant GM Jeff Kingston lead a committee to explore moving the fences soon after a Mariners road swing in late May. The Mariners had averaged seven runs on the trip and several players were frustrated they couldn’t hit like that at home.

Zduriencik and Kingston insisted the change wasn’t prompted by any one event. Zduriencik conceded that “the fact we started off slow probably got our attention” but added that the ballpark dimensions had been discussed internally for years.

“The two areas of the park that really stuck out to us as being outliers were left-center and center field,” Kingston said. “And more as a fly ball/home run type of batted ball. And really, the wind, the climate and the dimensions were all big factors to how balls play there.

“When we looked at the information relative to the other 29 parks, those were the two areas that really stood out,” he said. “Where we felt that we needed to make an adjustment.”

Fences will be moved in 12 feet in the left-center power alley and up to 17 feet between left-center and straightaway center. The left-field line is unchanged but will jut out only 6 feet instead of 10.

The hand-operated scoreboard atop the left-field fence will be moved, reducing clearance height for home runs from 16 feet to 8 feet. The right-center power alley and fence leading to dead-center will come in 4 feet.

The committee used batted-ball information from throughout the stadium’s history in deciding how far to move the fences. But not at numbers from this year because of a small sample size.

The Mariners entered Tuesday with a .218 batting average, .288 on-base percentage and .327 slugging percentage at home while scoring 3.0 runs per game. On the road, the Mariners hit .247 with a .300 on-base percentage and slugged .403 and scored 4.5 runs per game.

Kingston said the new dimensions still leave a park that slightly favors pitchers, but should increase home runs and scoring.

“You could make the argument that doubles and triples will go down because there is less outfield and the outfielders will play closer to home plate and catch more balls,” he said. “Our analysis still shows that the run environment is going to go up.”

That could hurt Seattle’s pitchers. But Zduriencik said it’s easier for pitchers to make small adjustments than for hitters to adapt swings.

Mariners starter Jason Vargas had a 4.78 earned-run average and 26 homers allowed on the road, compared to a 2.74 ERA and nine homers allowed at home.

“Now, a big power guy that just misses the ball, it still has a chance to go out,” Justin Smoak said. “Instead of getting caught at the wall, it goes out. There’s definitely a confidence booster with that.”

Mariners manager Eric Wedge said the change should be great for the team’s young hitters and potentially attract free agents.

“It’s all about just the ballpark playing fair,” he said.