A lot of talk has been around the recent earthquakes to hit the Bay Area. One of them ruptured an area in Contra Costa County where no faults had been detected before. Small quakes may have little impact, but if they occur on mapped or unmapped faults, they should serve as a warning, Schwartz said.

For more info, please read the original article by SF Chronicle:
https://www.sfchronicle.com/bayarea/article/In-California-hidden-earthquake-faults-lie-14536935.php