Hi All. As most of you know I do a lot of research into the various faults in and around the Bay Area. Most of what I find is on the net, but there are times I have to use the phone. The better half frowns on this, as sometimes the phone bill equals the gross national debt. I have a lot of “honey do’s” to do to pay for the phone bills.

Geologist and seismologist alike know that there is a major quake in the offering for the Bay Area. What they don’t know is when, and in some ways where. There are those who say where may not be important as when and now large as the whole of the Bay Area is going to be affected.

Scientists at the U.S. Geological Survey in Menlo Park say there is a 70% chance that a quake with a magnitude of 6.7 or greater will hit somewhere in the region in the next 30 years. There are those who say forget the 30 year time frame. It could be tomorrow when it strikes.

There are those who say the small quakes could be a precursor to an upcoming major quake, while others claim the quakes could be relieving some of the strain and actually delay the inevitable monster quake many scientists say will hit within the next 20 to 30 years.

"For the long range, we really don't know. We can't tell from the recent quakes in the North Bay whether the stresses are building up, or whether they're being relieved," said geologist David Schwartz of the U.S. Geological Survey.

Earthquake experts now are focusing on the Rodgers Creek Fault, (with a 32% chance for a major quake making it the number one fault in the Bay Area) which runs from San Pablo Bay through Sonoma County and has been hit with 14 small tremors in the last 32 years, Geologist David Schwartz of the Geological Survey told the San Francisco Chronicle.

Another hot spot is the Maacama Fault in northern Sonoma County, with five quakes recorded in the past five years, including two with magnitudes of 4.2 and 4.3, Schwartz said. I have been told not to view the Maacama fault as a separate fault, but as an extension of the Rodgers Creek fault and to view the Rodgers Creek fault as an extension of the Hayward fault.

Then there is the little Pinole fault in the bottom of San Pablo Bay. This fault is a thrust fault. Just recently there was a small swarm of quakes associated with this fault. Nothing came of that swarm. It is just another indication that a major quake is close by.

Repeated patterns of seismic activity in the Bay Area over the past two centuries point to the likelihood of a resumption of damaging earthquakes there in the near future, according to a new study done in 2000.

The author of the study, Tousson R. Toppozada, a leading seismologist with the state of California Division of Mines and Geology, believes that there is a strong possibility of quakes up to 6.5 magnitude sometime in the next few years.

"It is prudent to realize that we will experience activity in the magnitude 5.5 to 6.5 range again sometime in the near future," Toppozada said at a recent meeting of the Seismological Society of America in San Francisco "We really don't know how long the current quiescence in the San Francisco Bay Area can last, but we know it can't last forever. If the past is any indication of the future, we can expect some potentially damaging earthquakes within the decade, possibly by 2004."

Toppozada's theory is based on a review of seismic activity that shows that the four largest earthquakes in the Bay Area since 1838 were preceded by moderately strong quakes and followed by relative calm.

Scientists believe that the largest quakes--magnitude 7.0 and above--relieve stress and brought stability for a period of years. The experts say they do not fully understand the mechanics of seismic stress. But the record in the Bay Area indicates that eventually the pressure builds to the point where a series of moderately strong quakes culminate in a big one.

Toppozada said that the precursor shocks, usually about 6.0, signify that an area is vulnerable to a much larger seismic event. There have been a number of such quakes associated with the Calaveras fault.

The San Francisco quake of 1906, with an estimated magnitude of 7.8, was the largest studied by Toppozada. The 1906 quake was the last powerful jolt in the Bay Area for more than 60 years.

A 5.8 temblor in Santa Rosa in 1969 ended the period of quiescence. This would be the Rodgers Creek fault. Another quake is the 4.2 quake in Bennett Valley. This quake was on a thrust fault. There is another quake that hasn’t found a home yet. This is the 5.2 Yountville quake. This quake was a strike slip with just a hint of thrust.

There were similar, but shorter, periods of seismic quiet after the two other major events studied by Toppozada: the 7.4 earthquake centered south of San Jose in 1838 and the 7.0 quake at Hayward in the East Bay in 1868.

There has been a 12-year period of quiet since the Loma Prieta quake, but it is not expected to last as long as the calm that followed the 1906 quake.

Toppozada said that, in general, the larger the quake, the longer the subsequent period of seismic quiet. It stands to reason, he said, that the 7.8 San Francisco quake relieved more pressure than the 7.0 Loma Prieta quake.

Scientists with the U.S. Geological Survey said in 1985 that, based on 19th and 20th century quake records, they could predict, with 95% certainty, that a quake of about magnitude 6 would take place by early 1993 near Parkfield, on the San Andreas fault in Monterey County. No such quake has yet occurred. There are those who say that the quake did indeed occur. It just didn’t occur where they though it would. This is the Coalinga quake of 1983. Coalinga is about 12 miles to the east of Parkfield.

The historic record, however, does show that several major quakes in the Bay Area were preceded by a number of strong temblors. For instance, there were 10 quakes with magnitude 5.5 to 6.5 in the 13 years preceding the 1868 Hayward quake, 13 quakes in the 26 years before the 1906 San Francisco earthquake and seven in the 20 years leading up to the Loma Prieta quake.

"The pattern is evident, but certainly not a clear one," Toppozada said.

What this all boils down to is that the Bay Area is going to have a major quake. The number one fault to look at is the Rodgers Creek fault. This isn’t to say that the quake will occur on that fault, as there are other faults in the area that could support a major quake. However it is the fault that has the least amount of movement, as there is no evidence of creep in the southern portion of the fault. This is an area from about Rohnert Park south to San Pablo Bay. Will that major quake occur in this area? Only time will tell. Take Care…Don in creepy town