Thorsness won the 50 free in 25.97 seconds, out-touching Yoshi Andersen of Laguna Beach (25.97). Thorsness was third in the 100 free in 57.41.
The Costa Mesa boys and girls both finished second in league to Laguna Beach.
Estancia also has top swimmers at Thursday's Division III preliminaries, like senior Preston Schow. He was third in both the 50 free (23.47) and 100 free (51.62) at league finals.
The Eagles also have all three of their relays at CIF. The 200 medley relay team of Asher Twardowski, Conner Graham, Matt Thome and Schow qualified in 1:50.74, and the 200 free relay team of Graham, Derek Andrews, Thome and Schow qualified in 1:35.77. The 400 free relay team of Andrews, Twardowski, Brooks Watkins and Hayden Swift qualified in 3:41.14.
Other CIF qualifiers included the Costa Mesa girls' (Nicole Tfaye, Aubry Hill, Claudine Le and Thorsness) and boys' (Mason Cooper, Quinn Stone, Brian Lozano and Lewis) 200 free relay teams. The Mustangs' boys' 400 free relay team with the same quartet also qualified.
Stone (boys' 200 free), Tfaye (girls' 100 free), Sage Hill sophomore Jeremy Dorne (boys' 500 free) and Mesa's Tyler Connors (100 back) were also seeded for CIF preliminaries.
The top eight in each event advance to championship finals, and Nos. 9 to 16 to consolation finals Saturday at 10 a.m.