Hi All. Scientists study the past frequency of large earthquakes in order to determine the future likelihood of similar large shocks. For example, if a region has experienced four magnitude 7 or larger earthquakes during 200 years of recorded history, and if these shocks occurred randomly in time, then scientists would assign a 50 percent probability (that is, just as likely to happen as not to happen) to the occurrence of another magnitude 7 or larger quake in the region during the next 50 years.
But in many places, the assumption of random occurrence with time may not be true, because when strain is released along one part of the fault system, it may actually increase on another part. Four magnitude 6.8 or larger earthquakes and many magnitude 6 - 6.5 shocks occurred in the San Francisco Bay region during the 75 years between 1836 and 1911. For the next 68 years (until 1979), no earthquakes of magnitude 6 or larger occurred in the region. Beginning with a magnitude 6.0 shocks in 1979, the earthquake activity in the region increased dramatically; between 1979 and 1989, there were four magnitude 6 or greater earthquakes, including the magnitude 7.1 Loma Prieta earthquake. This clustering of earthquakes leads scientists to estimate that the probability of a magnitude 6.8 or larger earthquake occurring during the next 30 years in the San Francisco Bay region is about 67 percent (twice as likely as not).
With this in mind Petra set the task for me to find the fault that was most likely to be the source of the next major quake. She already knew, but wanted someone else to find it as a way of confirming her suspicions. What I found was that the Rodgers Creek fault has a 32% chance of being that fault which also happens to be greater then any other fault in the Bay Area. As the search for information continued I found that the one area most likely to be the epicenter of that earthquake is the section of fault between Rohnert Park and San Pablo Bay. The reason for thinking this is that there is no discernable creep in that portion of the fault and no written record of a major quake occurring on that portion of the fault with one possible exception. This exception is the 1868 Hayward earthquake. There is now some evidence although not complete that indicates the southern portion of the Hayward fault, or the southern portion of the Rodgers Creek fault could have caused the quake.
The rate of creep for the northern portion of the Hayward fault is such that it would put a jackrabbit to shame and with this in mind I think we are seeing some evidence in the strain build up. This would be the Rohnert Park earthquake, the Yountville earthquake, and a little pipsqueak of 3.0M+ quake near Vallejo. These quakes are the weak link in the chain. When there are no more weak links in the surrounding area there will be a major quake and it will be a lot sooner then anyone cares to think about. Take Care…Don in creepy town.