It might be a data collection problem--the JMA has deployed a pretty dense network of sensors over their entire territory, which makes sense given how geologically active it is.

In fact the Earthquake Early Warning's "next steps" plan says "New sensors need to be added in California to shorten the CISN sensors spacing to approximately 12 miles to facilitate timely EEW. The shorter the station spacing, the smaller the blind zone will be because warnings can be issued faster."

Check out this map comparison of the two networks: https://earthquake.usgs.gov/research/earlywarning/images/sta...

https://earthquake.usgs.gov/research/earlywarning/nextsteps....