California is well-know to be very geologically active, and has in the past experienced major earthquakes: 15 recorded since the mid-19th century above magnitude 7.0. Even a 6.0 earthquake can cause significant damage, and there are 47 listed in the same source.

The USGS maintains a comprehensive searchable data store of past earthquakes around the world. Occurrence of specific earthquakes is notoriously difficult. However, their statistics are fairly well-characterized over long timescales: a reasonable prediction can be obtained by simply dividing taking the number of 6.0 or greater earthquakes that have occurred in the last N years and dividing by N. (For example, the Wikipedia list has 39 since 1900.)

On April 11 a 5.2 magnitude earthquake 48 km from Bridgeport, California, according to earthquaketrack.com.

What will the magnitude of most severe earthquake to strike California in 2020 be?

This question resolves according to earthquaketrack.com. Any earthquakes with an epicenter fewer than 20km from the shore count towards positive resolution will count as having occurred in the state of California.

ETA 2020-05-08: The magnitudes are given on the moment magnitude scale.