Reality already thought of the answer, and congratulations, the west coast of North America is an example of it: tectonic sea level change.
So, North America is moving west relative to Africa. About two thirds of the west coast are active subduction zones. These are along Mexico, Cascadia, and Alaska. As other plates subduct under N. America, the plate is deformed and pushed up, just like the Andes in South America.
So why doesn't North America have an Andes mountain chain?
For reasons that weren't covered in my geology class, the west coast of North America can only be pushed up so high before something flat out cataclysmic happens - all of the land that was pushed back springs forward in a massive earthquake. If you, as a casual observer, measured where the sea level was before and after the quake, you would think that it had risen relative to the topography of the coast.
On your alternative Earth, a quake of truly mythic proportions began near San Diego. As the stored energy was released, the land under San Diego relaxed and both shot forward and lower than its previous elevation. This displacement caused a massive tsunami that rushes over the city, seemingly drowning it permanently. Another tsunami races across the Pacific, causing havoc thousands of miles away.
But it isn't over. The energy released also travels along the San Andreas fault, causing tectonic sea level change all the way to Northern California. There, the San Andreas intersects the Cascadia Fault. Offshore, the North American plate begins to spring forward over the Juan de Fuca plate. While the cities and people of California had no warning, those living in Portland and Seattle have about a half hour to evacuate before the tsunami hits.
It was never enough time.