Hi Kiddo. This isn’t the first time this has been seen, but it is the first time that it is being acknowledged. It was seen with the 1989 Loma Prieta quake, but not in the same way as the Denali quake.

As you know the Loma Prieta quake was 2/3 strike slip, 1/3 thrust. It has been speculated that the southern end of the Monte Vista thrust fault also moved during the quake. However there is nothing at the surface to indicate this.

Several studies identified the 27 June 1988 M=5.3 and 8 August 1989 M=5.4 Lake Elsman earthquakes as rare events that struck within 5 km of the future Loma Prieta rupture plane, and within 11 km from the 17 October 1989 M=7.0 Loma Prieta hypocenter. After both Lake Elsman earthquakes, the USGS and California State Office of Emergency Services issued a joint advisory of a heightened probability of M=6.5 shocks during the succeeding 5 days. The advisory was partly motivated by the observation that the two Lake Elsman events were among the three largest shocks to occur anywhere along the extent of the 1906 San Andreas rupture since 1914. Here we attempt to calculate the effect of the Lake Elsman shocks on the future Loma Prieta rupture.

We seek answers to the question, Did the Lake Elsman events hasten the occurrence of the Loma Prieta shock, influence the site of its nucleation, or its distribution of earthquake slip?

We find that the Lake Elsman events did not bring the future Loma Prieta hypocentral zone closer to failure. Instead, they are calculated to have unclamped the Loma Prieta rupture surface 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, an effect that could have been enhanced if the lowered normal stress permitted fluid infusion into the unclamped part of the fault. 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.

I believe this is the main reason I was told to watch for quakes less then 5 miles east of the Rodgers Creek Fault as they could cause the unclamping of the Rodgers Creek Fault. As you well know everything we have learned about this fault is that the southern segment of the fault isn’t moving while at the same time the northern Hayward Fault is moving at a pretty good clip. That creep is going somewhere and that somewhere could very well be the Rodgers Creek Fault. The area of the December 1981 swarm is an indication of weak spot. This could be the location of the next large quake on the Rodgers Creek Fault. If so you are going to get the ride of a lifetime. Take Care…Don in creepy town

• Loma Prieta Quake