A motorcyclist was fatally injured this morning in a multiple-vehicle freeway collision that snarled rush-hour traffic in the Miramar and Scripps Ranch areas, authorities said. The crash happened about 6:30 a.m. on southbound Interstate 15 just north of Miramar Way, California Highway Patrol public affairs Officer Jim Bettencourt said. Up to a half-dozen vehicles were involved, including a big rig and the motorcyclist, who was apparently hit by at least one vehicle after being thrown into the roadway. “The motorcyclist got underneath the semi, but I don’t know if he was ran over or what,” Bettencourt said, adding investigators were still finishing a preliminary report regarding the cause of the accident. A sedan stopped in the freeway behind the downed rider to ensure other vehicles would not hit him until emergency crews arrived. No other injuries were reported. Paramedics took the rider to Sharp Memorial Hospital, but he did not survive. The victim was a man, but no other identifying details were available. The paramedics told dispatchers on the way to the trauma center that despite CPR and other emergency treatment, the victim was not breathing after having suffered multiple head fractures. Witnesses told the CHP that after the rider was thrown from his motorcycle, he was hit by the semi and possibly other vehicles. Caltrans issued a SigAlert for southbound I-15 in the area, where several lanes were blocked and traffic was backed up to state Route 56 for several hours.