Hi All. I remember reading John Vidale’s report about how the Hector Mine quake affected the Landers fault.

"We were watching the Landers fault heal after it broke in 1992," Vidale said. "In measurements we took in 2000, after the Hector Mine earthquake, we saw that instead of continuing to heal, the rock had become less strong. We could actually see that the Landers fault had become weaker. Prior to that, we had four measurements from the five years before the Hector Mine earthquake that revealed progressive recovery -- each time stronger than the previous measurement. It was serendipitous that we captured a magnitude seven earthquake 10 kilometers away."

After reading his report I remembered something I had read in regards to the Loma Prieta quake. It seem this quake may have been triggered by two smaller quakes. If this is true there to a certain degree large quakes aren’t need to trigger an even large quake on a nearby fault. A study done by the following people indicate that the two quakes, 27 June 1988 M=5.3 and 8 August 1989 M=5.4 may have facilitated in the cause of the Loma Prieta quake

Hugo Perfettini Laboratoire de Géologie, Ecole Normale Supérieure, Paris

Ross S. Stein and Robert W. Simpson U.S. Geological Survey, Menlo Park

Massimo Cocco Istituto Nazionale di Geofisica, Rome

“We study the stress transferred by the 27 June 1988 M=5.3 and 8 August 1989 M=5.4 Lake Elsman earthquakes, the largest events to strike within 15 km of the future Loma Prieta rupture zone during 74 years before the 1989 M=6.9 Loma Prieta earthquake. We find that the first Lake Elsman event brought the rupture plane of the second event 0.3-1.6 bars (0.03-0.16 MPa) closer to Coulomb failure, but that the Lake Elsman events did not bring the future Loma Prieta hypocentral zone closer to failure. Instead, the Lake Elsman earthquakes are calculated to have reduced the normal stress on (or ‘unclamped’) the Loma Prieta rupture surface by 0.5-1.0 bars (0.05-0.10 MPa) at the site where the greatest slip subsequently occurred in the Loma Prieta earthquake. This association between the sites of peak unclamping and slip suggests that the Lake Elsman events did indeed influence the Loma Prieta rupture process. Unclamping the fault would have locally lowered the resistance to sliding. Such an effect could have been enhanced if the lowered normal stress permitted fluid infusion into the unclamped part of the fault. Although less well recorded, the ML=5.0 1964 and ML=5.3 1967 Corralitos events struck within 10 km of the southwest end of the future Loma Prieta rupture. No similar relationship between the normal stress change and subsequent Loma Prieta slip is observed, although the high-slip patch southwest of the Loma Prieta epicenter corresponds roughly to the site of calculated Coulomb stress increase for a low coefficient of friction. The Lake Elsman-Loma Prieta result is similar to that for the 1987 M=6.2 Elmore Ranch-M=6.7 Superstition Hills earthquakes, suggesting that foreshocks might influence the distribution of mainshock slip rather than the site of mainshock nucleation.”

This got me to thinking about some moderate past quakes in the Bay Area.

I’m sure there some who remember the two earthquakes of magnitudes 5.6 and 5.7 that shook the Santa Rosa area in 1969 and were felt generally as far away as the southern parts of San Francisco Bay. Another shock, 3.5, was registered in the interval between the two major ones, and two aftershocks, with magnitudes of 3.4 and 4.3, followed early the next morning.

The epicenters of these shocks were very closely grouped in an area about two miles north of Santa Rosa. The focal depths of the two main events were 9.6 and 10.4 kms. The smaller shocks were located with less accuracy than the larger ones, because fewer instruments in the University's network recorded the small earthquakes, but these seem to have been of the order of 10 kms deep.

Then of course there was the quake in the area of Bennett Valley. Then there was the quake near Yountville and now the most recent quake in the area of Santa Rosa. Not sure as to whether the Bolinas quake any affect on the surrounding faults. Not even really sure any of the other quakes has had an affect on the near by faults and it they did in what way did they?

It seems to me that what you learn on one fault could be used to determine the next failure of a fault. Unfortunately that doesn’t work with every fault. It may not even work for the same fault or the same location when the next large quake occurred. Take Care…Don in creepy town

• Lake Elesman