A memorial service for jockey Jorge Herrera, who was fatally injured in a racing accident Thursday at age 33, will be held Sunday at 10:30 a.m. in front of the racing office at the Alameda County Fair in Pleasanton, Calif. The fourth race on Sunday's closing-day card will be dedicated to Herrera.      Memorial donations in the rider's honor can be made by mailing checks to Racetrack Chaplaincy c/o Alameda County Fair, 4501 Pleasanton Ave., Pleasanton, Calif., 94566. The California Horse Racing Board announced it would conduct a thorough investigation into the fatal accident.