Like many of you, I’ve become curious about the Hermit Fault, since it both isn’t too far from our house, and has become quite a controversial subject — notwithstanding that I’d never heard of it until last year, and I don’t believe it every came up as an issue for any of the Westridge homes that might be on top of it.
For starters, I wondered where it got its name. It’s named after Hermit Ridge, which in turn is named after Domonico Grosso, a hermit who lived in the Jasper Ridge area from 1878-1915, although he apparently didn’t actually discover the fault. The fault was named by Willis in 1924, but he unfortunately didn’t leave a map or any details. Subsequent geologists like Page in 1989 have spoken about a fault there, but also unfortunately, didn’t provide many details.
The USGS has from time to time included the fault on its maps, and then removed it. For example: “No Hermit Fault is shown on this map owing to the lack of exposures demonstrating displaced strata or sufficient structural data or geomorphic features through which to draw a fault.” was in one of their reports.
Cutting to the chase, yes, it was on a 2010 fault map, and not on a 2017 fault map. In 2020 a geologist, Michael Angell, was hired to report on it, based on his prior work on the fault and some new field work. He also had access to the original 2017 work by Cornerstone Earth Group on behalf of Stanford assessing the fault, and concluded:
“The data presented in the draft Cornerstone (Earth Group) report do not provide direct or indirect evidence that can be used to make a reasonable judgement as to whether the Whiskey Hill/Ladera 11 contact on the site is depositional or a fault. Direct evidence to establish whether the contact is either depositional or a fault would require exposure of the contact at sufficient locations to permit direct observation and an informed judgment”
While some insist that 2020 was the last time anyone investigated the fault, fortunately, in 2021 Cornerstone Earth Group did exactly the sort of field work that was asked for, including digging a trench to a level carbon-dated back nearly 17,000 years, and having it peer reviewed by another firm. They found no activity, which clearly puts the fault in the non-active category, and considered safe for housing (the same rules that apply to any of us in Portola Valley).
A quote from their portion of the EIR: “A fault trace of the Hermit fault has been included on some maps as extending into the Project site. A trench investigation of the site to evaluate the potential presence of the Hermit fault, at the contact between the Whiskey Hill Formation and the Ladera Sandstone was performed by Cornerstone Earth Group (included as Appendix G).12 According to the letter report, the trenching did not expose the contact between the bedrock units on the site, and no active faulting was found in the trench walls exposed. The report states “the exposed geologic sequence within our trench consists of terrace deposits overlying a sequence of nearly vertical inclined beds belonging to the Ladera formation” (Cornerstone, 2021, page 9). The report states that no evidence of active faulting was found in the trench on the site. Radiometric carbon dating performed on the Terrace deposit, indicates a probable age of late Pleistocene, with a conventional radiocarbon age of 16890 +/- 50 years before present (BP). To be considered an active earthquake fault contact, the age of the Terrace deposits would have to be 11,000 BP or less and the contact would need to be a fault. According to the report, the Terrace unit is deposited over the Ladera Sandstone, indicating a depositional contact. This work was peer reviewed by Questa Engineering Corporation for this analysis and determined to establish that the suspected Hermit fault is not active in Holocene time and therefore is not an active fault.”