Our Seismologist Explains Why Bernal’s Chert Is Better Than Soft Rock During an Earthquake

The Surveyor 40/52

In pretty much every earthquake post that has appeared on this blog, I have extolled the virtues of Bernal’s beloved red chert because it doesn’t shake all that hard even during really big seismic events — especially compared to the marsh sand under the Mission or the artificial landfill in the Marina. Today is the anniversary of the 1989 Loma Prieta earthquake, so I figure this is a good to explain why chert keeps Bernal’s ground steadier.

Seismic waves travel at different speeds through different kinds of rock. The harder the rock, the faster the waves can travel. This alone is an advantage for shaking: waves spend less time traversing an area of hard rock than a comparably-sized area of sand or landfill.

Chert: 1    Landfill: 0

The issue is compounded, however, by the fact that every seismic wave has a specific amount of energy associated with it. If much of that energy goes into traveling quickly forward through the rock, less goes into shaking. But if the rock slows the forward propagation of the wave, the energy goes into shaking instead. The end result? Less shaking for less time in hard rock sites, more shaking for longer duration in soft soil sites.

Chert: 2     Landfill: 0

Liquefaction is another big problem with sand, soil, and landfill sites. Liquefaction is the process during which seismic shaking mixes loose soil and rock particles with groundwater, effectively turning the ground into quicksand. This was a huge problem in Japan and New Zealand this year, and it was also one of the main reasons the Marina was hit so hard in Loma Prieta. The more solid your rock, the fewer small particles there are to combine with groundwater, and the less the groundwater can permeate the rock in the first place. Bernal’s chert is good and solid, and it’s not going to turn to quicksand under us.

Chert: 3     Landfill: 0

So there you have it: Bernal’s chert means less shaking, for less time, without quicksand. In the event of another earthquake like Loma Prieta or 1906, Bernal Heights would certainly feel it, but our cherty geology will do a lot to help minimize the damage, whereas softer rock just strikes out.

PHOTO: Champi the Japanese Akita points out chert formations on Bernal Hill. Photo by Jay Axe

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27 thoughts on “Our Seismologist Explains Why Bernal’s Chert Is Better Than Soft Rock During an Earthquake

    • The geologic maps I have (which are from USGS) show that all of Bernal Heights is on Franciscan complex rocks, including the Cortland corridor. The Franciscan consists of a lot more than chert, true; there’s also old volcanic rocks, serpentine, and some metamorphosed sandstone.
      Sandstone, luckily, behaves differently than loose sand. It’s softer (and shakes a bit harder) than something like chert, but it’s still a solid rock and isn’t going to have the same amplified shaking or liquefaction problems as unconsolidated marsh sands or artificial fill.

  1. this is, ov course, asuming that the quake doesn’t rupture the dodgy gas pipes blowing the whole of Bernal off the map forever :)

  2. I’m a lifelong chert fan, especially of Bernal Hill’s warmly charming variety. But I must say that the Franciscan chert of Glen Canyon seems more… mysterious… prehistoric… something… In any case, my dog looks darling frolicking around both, so the more chert the better!

  3. I think Bernalwood is a very cute name for a blog, and the imitation Hollywood sign makes a cute graphic.

    However, I think it’s a horrible name for a neighborhood, as unattractive as a Hollywood sign would be on top of Bernal Hill.

    I like living in Bernal HEIGHTS, which (thank heaven) bears little resemblance to Hollywood, and I beg the writers who keep referring to our neighborhood as BernalWOOD in their posts and comments to reconsider.

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