Major Asperities that Ruptured in Large California Earthquakes, Surrounding Pre-Shocks, and Some Yearly to Decadal Precursory Changes in Seismic Activity

Double-difference locations of forerunning shocks of seismic magnitude, M, 2 to 6 are examined in the months to decades before 11 mainshocks in California of magnitude 6 and larger. Each of the 11 had large quiet zones beforehand, which are called asperities, that break nearly entirely in large mainshocks. Their surrounding 11 zones were all sites of small to moderate-size shocks that define the approximate magnitudes of the coming large event. The latter, donuts of activity, are places to examine for precursory changes years to decades before larger earthquakes. The quiet asperities and many sites at the earth’s surface are not good places to monitor precursory changes before large mainshocks. Detecting forerunning events to large future earthquakes requires monitoring the right places. A few possible precursory changes are identified and discussed.