In the past 24 hours there have been 23 small quakes in the San Ramon area on the Pleasanton fault, with magnitudes ranging from 0.8 to 3.0, and hypocenter depths around 8 km.
Immediately friends began contacting me asking what this means. There’s good news and bad news.
The good news is that the San Andreas fault complex (including the Hayward, Calaveras, Pleasanton faults , etc) does produce periodic clusters of quakes like this, without the clusters necessarily meaning a larger quake in imminent.
The bad news is that such clusters also remind us that the stress in the rocks of the Bay Area is continually growing and shifting. So, in the long run, clusters like this are indications that the “Big One” is going to happen. So are the smaller, less-clustered quakes we get elsewhere on a daily basis. A cluster like this probably means that the tension beneath the surface is realigning, shifting in some new way. Such shifts are not necessarily good—one area gets less stress, another area gets more stress. And maybe that new additional stress is just the straw on the proverbial camel’s back needed to trigger a quake.
A problem is that we cannot tell how such shifts affect other faults. The Pleasanton fault is not thought to be a major player in earthquake hazards in the Bay Area—though there’s always the possibility it could surprise us. The shifting of stress could, however, put more stress on known culprits such as the Calaveras and Hayward faults.
The upshot of this San Ramon cluster is: Don’t Panic. It will probably just go away. But the upshot is also this: Prepare Now. Because this cluster is a reminder that the “Big One” is coming…