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.)
Will (at least one) magnitude 6.0 or greater earthquake strike California in 2020?
This question resolves according to credible earthquaketrack.com. An earthquake with an epicenter fewer than 20km from the shore count towards positive resolution.