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

About these ads
This entry was posted in Bernal Hill, Geology, Geology and Seismology. Bookmark the permalink.

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

  1. Erik says:

    that’s why, during the ’89 quake, I only spilled my tea.

  2. SER says:

    My dog also likes to point out chert formations. All hail chert!

  3. Derek says:

    Praise chert. But if you have an older house it’s still smart to make sure your house is bolted to the foundation and has sheer walls in the garage. It is not that expensive and can really reduce damage to your house.

    http://www.bayarearetrofit.com/faqs.html

  4. jayaxe04 says:

    Cool, I learned something new about Bernal’s Chert. Way to go Champi!

    Thanks for the credits too.
    -Jay

  5. sally says:

    not all of bernal is chert. a lot is sand (like all of cortland). how will that fare?

    • Julian Lozos says:

      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.

  6. why says:

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

  7. ray says:

    this is chertainly good news for our beloved bernal!

  8. friscolex says:

    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!

  9. 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.

  10. Janet C says:

    Yeah, but Bernalwood is a much catchier blog name.

  11. Almost a perfectly timed post…did you know????

  12. Pingback: Champi Headlines another Blog! « Jay Axe's Photo Blog

  13. Pingback: Bernalmorphism: Why Our Seismologist Draws Bernal Heights as a Dog | Bernalwood

  14. Pingback: Young Child Discovers the Joy of Bernal’s Chert | Bernalwood

  15. Pingback: We Are Bernal Heights. We Are The People of the Chert | Bernalwood

  16. Pingback: We Are Bernal Heights. We Are The People of the Chert | Bernalwood

  17. Pingback: Final Plan for Bernal Hill Trail Restoration Unveiled | Bernalwood

  18. Pingback: This Is Only a Test: NERT Emergency Preparedness Drill in Precita Park on Saturday | Bernalwood

  19. Pingback: What You Missed When You Missed Glenn Lym’s Talk About the Lost Geology of Bernal Heights | Bernalwood

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s