Hi All. While there are no universally recognized patterns of seismicity seen preceding mainshocks, one pattern that has been seen in several different parts of the world can be summed up generally by the following:
Seismicity increases over its normal rate in a particular area -- that is, you will see more earthquakes per day than usual. The seismicity in that area will then come to a stop, fairly quickly. A lull sets in, with about a month or two of relative seismic "quiet" (low rate of earthquakes). Then, just before the mainshock strikes (1 to 10 days roughly although it could also be years), seismicity increases suddenly in the center of the area, which showed increased seismicity before. A foreshock may precede that mainshock.
The cause of this pattern is thought to be related to a high level of stress along a large fault. This high stress causes smaller, weaker, and adjacent faults along the edge of the impending rupture zone to break, leading to an increase in seismicity in a particular area. Once all the minor faults have ruptured, the activity stops, and nothing but the primary fault remains unbroken. That fault, then, shoulders the stress entirely by itself, and after straining quietly for a short time, finally begins to give, eventually rupturing in a large earthquake.
This is what were seeing now in the North Bay Area. First there was the Bolinas quake, then the Rohnert Park quake and last (for the time being) the Yountville quake. We could also throw in the two Santa Rosa quakes in 1969. There is a good amount of creep on the northern section of the Hayward fault. This creep is being absorbed somewhere and when that area can no longer absorb that creep something will have to give. The area most likely to give at this moment is the Rodgers Creek fault. Most likely the southern portion of it as there is no indication of movement on it. The next likely area is the southern portion of the Hayward as it plays catch up with the northern part.
However there is one problem with this theory. How do we identify that quiet period? Will there be small quakes along the fault before it breaks, or will it break with one large quake? Remember. I’m talking about the Rodgers Creek fault that lies between Rohnert Park and the San Pablo Bay. Take Care…Don in creepy town.