Earthquake activity: Half Moon Bay-area historical earthquake activity is significantly above California state average. It is 7247% greater than the overall U.S. average. On 4/18/1906 at 13:12:21, a magnitude 7.9 (7.9 UK, Class: Major, Intensity: VIII - XII) earthquake occurred 48.5 miles away from Half Moon Bay center, causing $524,000,000 total damage On 10/18/1989 at 00:04:15, a