Hi Steve. I don’t know the mechanism of the two foreshocks to the Loma Prieta quake, but I do know some people who might so I will give them a call tomorrow. I’m not even sure if there are any as the “moment tensor solutions” weren’t that widely used at that time.

The quakes you’re referring to didn’t occur near Lexington Lake. They occurred near Lake Elsman.

On June 27, 1988, and August 8, 1989, there were two earthquakes of magnitude 5.0 and 5.2 near the Santa Cruz Mountains segment of the San Andreas fault. In both cases, scientists were concerned that these events could be foreshocks to a larger earthquake because of their magnitude and their position. Foreshocks do occur less than 5 days before about half of the large earthquakes in California. For these reasons, the California Office of Emergency Services issued an advisory of an increased likelihood of a major earthquake within the next 5 days following those quakes. The Loma Prieta earthquake of October 17, 1989, was a little late, but it was the quake anticipated by the advisories.

It was found 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.

On April 4, 1990, a magnitude 4.5 earthquake shook the region near Walnut Creek, California. Scientists were concerned that this earthquake could be a foreshock to a magnitude 6.5 quake on the Calaveras fault. They decided, however, that this event, and two others like it on April 27, was probably not foreshocks because of their location and the sequence of the many smaller earthquakes that accompanied them. No advisory was issued and no major earthquake has hit yet in this region.

As you can see this foreshock thing can be quite controversial at times. This brings to mind the past earthquakes in the Bolinas, Bennett Valley and Yountville areas. Are they foreshocks to future quakes, or just the common ordinary run of the mill quakes? How about the swarm of quakes that occurred just east of Petaluma in Dec. of 1981? The largest quake in that swarm was just a little over M>3.0+. Take Care…Don in creepy town