Hi All. The Crystal Springs ELF data is in the middle, or very near the middle of the section of the San Andreas fault between Portola Valley and San Francisco that has had nothing in way of earthquakes since the great quake of 1906.

In the great earthquake of April 1906, which began near 38.03N/122.78W and propagated out both North and South 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. This was the highest risk assigned to any fault segment in the Bay Area. This is the area of the 1989 Loma Prieta quake. 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.

What about even farther South? The risk on the part of the fault South of San Juan Bautista all the way down to just North of Parkfield is considered low. Along that section the San Andreas seems to continually creep, constantly releasing strain and thus not storing enough energy for a major earthquake.

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? 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.

This brings us to the peninsula section of the San Andreas fault? It has moved less than areas farther North in 1906. Will it also produce a major earthquake sometime soon? Is there another “seismic gap” between Portola Valley and San Francisco? Is the ELF data at Crystal Springs indicating a major quake for the Bay Area is on the way? Lots of questions, but not many answers. Take Care…Don in creepy town