Well Seth Clearwater has not been casted yet. But to find out who has been casted you can go to this website: http://www.imdb.com/title/tt1259571/fullcredits
His name is Booboo Stewart and portrays the character Seth Clearwater in New Moon and Eclipse.
BooBoo Stewart (no relation to Kristen Stewart) is playing Seth in the Eclipse movie. (probably in Breaking Dawn also)
Tyler Posey is rumored to be playing Seth in New Moon
In the pilot he was played by Darin Heames, but now in season 2 John Corbett (who you might know as Aidan from SATC) will be playing Seth.
Seth Scott is an NBA professional player playing for Atletico's de San German. He was born in Orem, Utah in the United States of America.
Seth Greisinger debuted on June 3, 1998, playing for the Detroit Tigers at Tiger Stadium; he played his final game on June 5, 2005, playing for the Atlanta Braves at Turner Field.
strangle hold by ted nugent echoes by The Rapture
Seth Morehead debuted on April 27, 1957, playing for the Philadelphia Phillies at Connie Mack Stadium; he played his final game on September 15, 1961, playing for the Milwaukee Braves at County Stadium.
Seth McClung debuted on March 31, 2003, playing for the Tampa Bay Devil Rays at Tropicana Field; he played his final game on October 4, 2009, playing for the Milwaukee Brewers at Miller Park.
Seth Etherton debuted on May 26, 2000, playing for the Anaheim Angels at Edison International Field; he played his final game on June 3, 2006, playing for the Kansas City Royals at Kauffman Stadium.
BooBoo Stewart will play Seth Clearwater, Julia Jones will play Leah Clearwaterhttp://www.imdb.com/title/tt1325004/fullcredits#cast
Boo Boo Stewart. He was in the musical group T-Squad