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The Case of the Missing Earthquake Fault

Portola Valley straddles two tectonic plates (the North America Plate and the Pacific Plate). This region is one of the most seismically active in the World, and Life on the San Andreas Fault (the title of Town Historian and author Nancy Lund’s history of Portola Valley) would be a much riskier undertaking were it not for the founding citizenry, many of whom were professional scientists who volunteered their time and expertise to pioneer the comparatively safe development of geologically hazardous lands.  The building standards they developed right here in Portola Valley for seismically safe construction and risk mitigation were later adopted by other municipalities and by the State of California.

The first step in mitigating the risk of death and destruction from an earthquake is knowing where the hazards are located. The San Andreas Fault has been well mapped and studied, and is visible to the naked eye at various locations in town (for those who know what to look for). And though it is the most hazardous fault, it is not the only seismic hazard residents face. There are other, smaller faults which could be sources of potentially devastating energy release. One such fault is the Hermit Fault (see below).

The Safety Element of our General Plan (2010 version, section 4143) states, in part, that the official Town Geology Map, maintained by the Town Geologist, shall:

Show active and potentially active faults on the town Geologic Map and Ground Movement Potential Map. On the Ground Movement Potential Map show required setbacks for buildings for human occupancy and add corresponding provisions to the zoning ordinance.” (our emphasis).

The 2010 revision of the Geologic Map, Town of Portola Valley, which was adopted by resolution and signed by then Mayor Toben, shows a fault known as the Hermit Fault, which crosses the Stanford Wedge, a Stanford University owned property which Stanford is actively seeking to develop.

The 2017 revision of the map (also available here), adopted by TC resolution, does not show the Hermit Fault. This is very strange, because according to a 2020 report by geologic consultant Michael Angell, who mapped the fault in 1995 and 1996, (findings from original field work available here; see pg 52) the evidence by him and others is conclusive that the fault exists and is potentially active.

2010 Map Wedge Detail.png

2010 version of the Geologic Map clearly shows the Hermit Fault.

2017 Map Wedge Detail.png

2017 version of the Geologic Map mysteriously does not show the Hermit Fault.

There has been no explanation for the discrepancy between the 2010 and 2017 map revisions. According to minutes of the January 20, 2021 Planning Commission meeting on the proposed Stanford development project, resident, professional geologist and member of the Geological Safety Committee, Nan Shostak, stated the following during public comment:

"Hi! Thank you for the opportunity to comment. I’d like to address what may be a fundamental issue for the Wedge project. I’m Nan Shostak, resident of Portola Valley and geologist, retired from teaching geology at San Jose State. I’ve spent years researching the local geology and 1906 earthquake here and wrote a Master’s thesis on the 1906 earthquake in San Jose. I think all the Commissioners have, by now, received my recent short paper on the difficulty or impossibility of safely evacuating Portola Valley after large earthquakes; I hope you’ll read it.

The founders of our town had the wisdom to recognize the seismic hazard and risk of living in this seismically active area, and they had the foresight to make geologic hazard maps for the town. Of particular concern right now is the Hermit Fault. The Hermit Fault has a long, well-documented history, confirmed by all the geologists who’ve mapped this area since 1924.It was on our Geologic Hazard and Ground Movement Potential maps until 2017, when it was inexplicably removed. I can find no report from either the Planning Commission or the Council to explain that removal. The Council adopted a policy approving other changes to these maps, but there is no mention of the Hermit Fault. It simply disappeared from the maps.

The Hermit Fault is shown on all except the Portola Valley maps as crossing through Stanford’s proposed building site as well as Alpine Road. In my short paper, I note that if a large earthquake on the San Andreas triggers movement on the Hermit Fault, Alpine Road at the Wedge will likely be impassable until extensive repairs have been made. Further, there is a heightened risk of wildfire ignition and spread, if housing is allowed to be developed on and near a potentially seismically hazardous geologic fault, as Stanford proposes.

Part of the mapped fault lies directly under some of the houses on Stanford’s site plan. Stanford’s own geotechnical report found indisputable evidence of a fault in the middle of the proposed development site. On Red page 50 of tonight’s packet for this meeting, our Town Geologists at Cotton, Shires say this fault should be located by an on-the-ground investigation, its geologic character should be determined, and its potential for seismic risk to the houses proposed for the site should be evaluated.

 We must question the wisdom of building housing for families near and immediately above this long-known geologic fault whose potential seismic activity is unknown and hasn’t been investigated. A thorough, independent, on-site investigation of the Hermit Fault on the Stanford Wedge should be completed before development plans are considered further.

 Thank you."

In the same meeting, when asked about the fault, Planning & Building Director Laura Russell told the committee that “they’ve been doing research with the Town Geologist regarding the history of that fault and what’s known about it, and then there will be a lot more information to be covered later.” Apparently, these things move slower than continental drift, because the residents are still waiting. Wouldn't it have been more prudent to do the background research before removing the fault from the map?

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