Hi All. San Andreas Lake (from which the fault takes its name) is a ``sag pond'' that naturally formed in the valley of the San Andreas fault. Strike slip faults are good places for lakes; the fault both creates a low spot to collect the water, and grinds up the rock underneath making an impermeable layer to hold the water in.

If you were to extend the line of the lakes to the horizon you would find 3000-foot tall Black Mountain located just West of Palo Alto and Stanford University astride the fault. The fault changes orientation at Black Mountain by about 9 degrees, and the land on either side is being crumpled and uplifted as it gets dragged around the corner.

In the great earthquake of April 1906 the displacement was up to 21 feet in the Point Reyes area, the largest surface displacement ever recorded. Along the San Francisco Peninsula section of the fault the displacement was less, and it dropped yet again South of Black Mountain. Some geophysicists speculate that this might have happened because the moving rupture partially dissipated when it hit the obstruction.

On the theory that the great 1906 earthquake did not dissipate much of the accumulated strain South of Black Mountain (as it presumably did North of San Francisco), the USGS in 1988 assigned a relatively high risk (30% over the next 30 years) of a major earthquake along the Santa Cruz Mountains section of the San Andreas, between Black Mountain and San Juan Bautista.

If you drive up Page Mill Road from Palo Alto, the main trace of the San Andreas fault is noticeable as an abrupt dip in the road just before you start up ``three grunt hill'', a short steep stretch of road. This stretch of road is infamous among bicyclists. When one is on top of Black Mountain they can see what seems forever. The highest mountain visible in the distance is Mt. Umunhum. On a clear day an even higher mountain, Loma Prieta, will be visible slightly above and to the right of Umunhum. Loma Prieta is the location near where the October 17, 1989 "World Series" earthquake rupture began. This quake happened right in the middle of the stretch of the San Andreas fault previously identified as ``high risk'', on the basis that there had been a persistent ``gap'' in the earthquake activity there. The Loma Prieta earthquake rupture spread both North and South from the epicenter, filling most of the previously noted gap, but stopped well short of reaching Black Mountain.

This leads to the obvious question... is there enough strain to power another significant Bay Area earthquake yet stored along the stretch of the San Andreas around Black Mountain? And was the Loma Prieta earthquake really the one that the USGS had expected? There is some disagreement about that, because the aftershocks map out a fault plane dipping at about 70 degrees to the South West, not a vertical one as had been expected for the San Andreas, and the movement was a combination of about 2/3 strike slip and 1/3 thrust faulting, not purely strike slip.

What about the peninsula section of the San Andreas fault? It also moved less than areas farther North in 1906, and the USGS in their 1988 report gave it a probability of 20% of producing a major earthquake over the next 30 years.

Is there another ``seismic gap'' between Portola Valley and San Francisco? Just recently (little more then a month ago) there was a small swarm of quakes near Portola Valley. Could this be the location of the next big quake on the San Andreas fault?

For those living in the Bay Area I have to recommend that a field trip to Black Mountain would be well worth the time and effort. Just downhill from Monte Bello Open Space Preserve on the left (West) and Los Trancos Open Space Preserve on the right (East). At the entrance to both of these preserves there are free trail guides that point out features associated with the fault: sag ponds, push-up ridges, long straight valleys, fences offset in 1906, etc. In particular, just inside Monte Bello Open Space Preserve note the long skinny cattail pond next to the trail up Black Mountain. That's another sag pond trapped by the San Andreas fault, a miniature version of San Andreas Lake. It's quite astonishing during summer in this otherwise bone-dry area to come upon a little pond complete with frogs and cattails!

A though just came to mind. The “Boss Lady” Petra hasn’t been out of the house on a field trip for a good spell now. With working on her web site and taking care of general maintenance around the house she hasn’t taken any time for herself. If I promise not to take the turns on two wheels and drive at a reasonable speed down the highway would you help me in convincing her to go on a field trip to Black Mountain? Please. Take Care…Don in creepy town.