The plate rate at Sumatra is about the same as across California. So, should be about the same number of quakes....but right now we are in a period of a ton of aftershocks. The Nation of Indonesia is spread over a much larger area than Californoa and has different plate boundaries, so would have greater overall seismicity, but over a larger area. Indonesia has subduction zones and California only has one in the far north, so quake magnitudes can be bigger in Indonesia.
Chris