Why isn't there a huge deep sea trench next to California like there is next to South America?

Deep-sea trenches generally lie seaward of and parallel to adjacent island arcs or mountain ranges of the continental margins. They are closely associated with and found in subduction zones—that is, locations where a lithospheric plate bearing oceanic crust slides down into the upper mantle under the force of gravity.

Explanation:

"The result is a topographic depression where the oceanic plate comes in contact with the overriding plate, which may be either oceanic or continental. If the overriding plate is oceanic, an island arc develops. The trench forms an arc in plan view, and islands with explosive volcanoes develop on the overriding plate. If the overriding plate is continental, a marginal trench forms where the topographic depression appears to follow the outline of the continental margin. Explosive volcanoes are found there too"

hi there!

the answer you are looking for is relatively steep.

have a nice day!