RIO DELL, Calif., Dec 20 (Reuters) - A magnitude 6.4 earthquake jolted the extreme northern coast of California before dawn on Tuesday, injuring at least two people, damaging roads, bridges and power lines and leaving tens of thousands of homes and businesses without electricity.Tuesday's temblor set off one structure fire, which was quickly extinguished, and caused two other buildings to collapse, according to the California Department of Forestry and Fire Protection (CalFire).One of those victims was a child with a head injury and the other an older person with a broken hip, according to local media reports citing the sheriff's office.Police closed the main bridge crossing the Eel River into Ferndale, a picturesque town notable for its gingerbread-style Victorian storefronts and homes, after four large cracks were discovered in the span.The highway patrol reported at least four other roads in Humboldt County closed due to earthquake damage, and a possible gas line rupture under investigation."