A really large amount goes through Canada. The port of Vancouver is bigger than Oakland, and the port of Manzanillo Mexico is only slightly smaller. Both ports have rail connections to the US interior. The Mexican ports have lower market share because they are pretty far from Chinese origins, but if you want to amuse yourself you can Google up a lot of crazy conspiracy theories about the connections between Mexican ports and the US city of Kansas City. Many deranged people believe that the Mexicans are usurping our sovereignty via an obscure customs office in Kansas City that is (barely) connected by rail to the port of Lazaro Cardenas.