Over the last three years, baseball has seen massive changes in its offensive environment. The launch angle revolution has certainly played a part in a few offensive breakouts but no factor has been as impactful as the ball itself. Yesterday, I took a look at the league-wide ramifications the dead(er) ball has had on offense this year. Coincidentally, Craig Edwards posted a far more in-depth study on FanGraphs yesterday as well. The conclusions were the same: the physical characteristics of the ball have changed again and it’s suppressed offensive production league-wide.

In 2016, Safeco Field saw the most home runs hit out of any ballpark in baseball. For years, Safeco had been a pitcher’s haven, a place where the deep left field power alley and the heavy marine layer subdued thousands of fly balls. But part way through 2015, something in the baseball changed to give it a little less air resistance. That led, in part, to a massive explosion in power across baseball, and Safeco Field saw a significant flip in those familiar park effects. Observe:

Safeco Field Hard Contact 2015–2018 Year Avg. Exit Velo (mph) Avg. Fly Ball Distance (ft) HR/FB% ISO wOBA-xwOBA Year Avg. Exit Velo (mph) Avg. Fly Ball Distance (ft) HR/FB% ISO wOBA-xwOBA 2015 88.2 313 15.6% 0.150 -0.004 2016 88.5 319 18.6% 0.177 -0.004 2017 87.3 315 15.5% 0.167 -0.010 2018 88.0 315 14.5% 0.155 -0.022

In 2016, the overall home-run-per-fly-ball rate spiked to 18% in Safeco Field. That’s a very high home run rate—for reference, Coors Field ran a 22% home-run-per-fly-ball rate in 2016. The homer friendly trend dropped back to 2015 levels last year but the “juiced” ball was still cutting right through the marine layer. This season, the home run rate has dropped again, closer to where it was prior to 2015. Looking at the expected wOBA differential confirms what we saw in yesterday’s posts: the ball just isn’t flying as far, particularly on hard contact.

The Mariners pitching staff has thrived as a result of this new offensive environment. They’ve allowed an 11.3% home run rate at home this season, a significant improvement over the 14.1% mark they allowed in 2016. And this isn’t a coincidence either. I’ve written at length about the way Jerry Dipoto has built his pitching staff since becoming GM in 2015. He built a fly ball heavy pitching staff to take advantage of the environment he thought would carry over from Safeco’s early life. I don’t think he could have anticipated the very physical characteristics of the baseball throwing that plan out the window.

With the offensive environment swinging back to where it was before this home run boom, those same pitchers Dipoto acquired because of their fly ball tendencies are mostly thriving this season. The best example is Marco Gonzales.

Marco’s health, his new pitch repertoire, and his maturity have all contributed to his breakout season. But he’s also cut his home run rate by seven points, all the way down to 10.6%, despite allowing far more hard contact than last year. The average exit velocity on the fly balls and line drives he’s allowed is higher, and opposing batters are pulling the ball almost half the time they put the ball in play off him. And yet he’s well outperforming his xwOBA on those same batted ball types.

Marco Gonzales Hard Contact Indicators Year HR/FB% FB+LD Exit Velo (mph) FB+LD wOBA-xwOBA Year HR/FB% FB+LD Exit Velo (mph) FB+LD wOBA-xwOBA 2017 17.8% 92.3 0.064 2018 10.6% 93.6 -0.085

In fact, if you look at how the entire starting rotation has performed on fly balls and line drives, they’re all outperforming their xwOBA by significant margins this year. That’s allowed Gonzales and Wade LeBlanc and other fly ball heavy pitchers on staff to pound the zone without worrying about giving up cheap home runs.

The dead ball hasn’t been a complete miracle. Part of the equation Dipoto built relied on strong outfield defense to run down all those fly balls. The outfield just hasn’t been as defensively sound as it was last year and so more of those fly balls and line drives are falling in for hits. Their defensive efficiency on fly balls has fallen from over 90% in 2017—the best in the league—to a middle-of-the-pack 88%.

Luckily, the Mariners offense hasn’t really suffered any ill effects from the dead ball. Their home-run-per-fly-ball rate is basically the same as last year but they rank much higher among major league clubs because the rest of the league has taken a step back.

With Safeco Field trending back towards a pitcher-friendly environment, the Mariners are thriving. A season ago, they allowed 4.4 runs per game at home. That number has dropped to 3.8 runs per game in 2018. It’s a big reason why they’ve been able to rack up win after win despite an offense that’s started to sputter without Robinson Canó. And barring any drastic changes to the baseball mid-season, the new dead ball should continue to give them life.