The deal, which was announced Monday, involves multiple players and required Segura to waive his no-trade clause to join Philadelphia.
Segura, 28, batted .304 with 10 home runs, 91 runs and 63 RBIs in 144 games last season and was named an American League All-Star.
In June 2017, Segura agreed to a five-year, $70 million contract with the Mariners spanning 2018 to 2022. The deal has a $17 million option for 2023, with a $1 million buyout.
Segura was the centerpiece of one of Seattle's biggest moves after the 2016 season, when he was acquired from the Arizona Diamondbacks as part of a five-player trade.
He will become the latest high-profile player traded away by the Mariners, who are tearing down their roster in an effort to rebuild the team.
Santana, 32, signed a three-year, $60 million contract with the Phillies last offseason and has a base salary of $17 million in 2019 and $17.5 million in 2020. His contract has a club option for the 2021 season worth $17.5 million with a $500,000 buyout.
He hit .229 with 24 home runs and 86 RBIs in 2018.
The 23-year-old Crawford, drafted by the Phillies in the first round of the 2013 draft, hit .214 with three home runs and 12 RBIs in 49 games last season.
The left-handed Pazos, 27, was 4-1 with a 2.88 ERA in 50 relief innings last season. Nicasio, a 32-year-old right-hander, was 1-6 with a 6.00 ERA and one save in 42 relief innings in 2018. He pitched for the Phillies briefly in 2017 after they claimed him off waivers from the Pittsburgh Pirates and later traded him to the St. Louis Cardinals.
With Santana traded, Rhys Hoskins will return to first base for the Phillies after playing in left field last season.