After That Earthquake, Have You Hugged Bernal’s Chert Today?

chertislove2

Last weekend’s dramatic 6.0 earthquake was centered around American Canyon, but it caused substantial damage in the nearby town of Napa. Get well soon, drinky Wine Country neighbors!

But did you feel the quake here in Bernal when the earth shook at 3:20 am? The answer to that question seems to vary depending on a) the precise location of your home, and b) how heavy (or light) a sleeper you are, and c) if you have dogs.

Regardless, this earthquake provided a vivid demonstration of the importance of personal earthquake preparedness, and even better, NERT training.

It also provides yet another opportunity for all Bernalese to give thanks for the blessed chert that has endowed our neighborhood with such a stable foundation of earthquake-resistant bedrock. As Julian Lozos, our Senior Seismologist, previously explained:

Geologically speaking, Bernalwood is actually closer to the San Andreas than downtown, but the solid chert bedrock that makes up Bernal Hill didn’t shake nearly as hard as the soft sediment and artificial fill of the Financial District, Mission, or SoMa. That same chert explains why Bernal residents often miss smaller quakes that rattle people in other parts of the City.

(SIDE NOTE: If you’re not following Julian on Twitter, you’re missing his awesome guided tour of last weekend’s quake.)

But what exactly is this blessed chert, for which we all should be so grateful? Where did it come from? And how did it end up in Bernal Heights? Here’s a basic 411 on chert from Gelology.com:

Chert can form when microcrystals of silicon dioxide grow within soft sediments that will become limestone or chalk. In these sediments, enormous numbers of silicon dioxide microcrystals grow into irregularly-shaped nodules or concretions as dissolved silica is transported to the formation site by the movement of ground water. If the nodules or concretions are numerous they can enlarge and merge with one another to form a nearly continuous layer of chert within the sediment mass. Chert formed in this manner is a chemical sedimentary rock.

Some of the silicon dioxide in chert is thought to have a biological origin. In some oceans and shallow seas large numbers of diatoms and radiolarians live in the water. These organisms have a glassy silica skeleton. Some sponges also produce “spicules” that are composed of silica. When these organisms die their silica skeletons fall to the bottom, dissolve, recrystallize and might become part of a chert nodule or chert layer. Chert formed in this way could be considered a biological sedimentary rock.

Bernal’s chert is a local type called (…wait for it...) Franciscan chert, and Franciscan chert comes from compacted sediments formed by zillions and zillions of  tiny protozoa critter skeletons. Over the course of zillions and zillions of years, these sentiments hardened into layers on the bottom of the ocean, and today those layers are clearly visible in the cross-section of our chert.

The Wikipedia page for Bernal Hill explains how our chert became our hill, and why it’s that stylish reddish color:

Bernal Hill, along with the other hills in the San Francisco area, is a folded hill, created by the “wrinkling up” effect of the Pacific plate subducting under the North American plate, when the North American and Pacific plates were converging, around 150 million years ago. Near the summit you will find folded layers of very hard rock called radiolarian chert. It is a sedimentary sillicate rock which gets its sillica content from the shells of microscopic creatures called radiolaria. The red color comes from iron oxide.

So that’s how Bernal ended up with all our chert, and how it got its coloring. And here’s how our chert is distributed, as seen through the spiffy Google Earth Geology layer:

bernalgeology4

 

The red areas are Franciscan chert, purple is Franciscan volcanic rock, green is Franciscan serpentine rock, blue is Great Valley serpentine rock, and yellow is rock fragments in the form of hillslope deposits. The yellow-gray and lighter yellow are alluvium soil. The light gray is (eek!) artificial fill.

Here’s the reverse angle, looking at Bernal Hill from the north:

Bernalgeology.northview

As Julian explained a little while back, our beloved chert anchors Bernal Heights and absorbs much of the energy created by seismic waves.  So if you happen to be on Bernal Hill in the next few days, go ahead and find one of our rakish exposed chert formations. Then, approach the chert reverently, and give it a big wet kiss. Someday, the home that chert saves could be your own.

PHOTO: Chert on Bernal Hill, by Telstar Logistics

We Are Bernal Heights. We Are The People of the Chert

We are Bernal Heights.

We are the People of the Chert.

We are the People of the Chert, and we are growing larger! In the remote terrain high above San Francisco, @willak captured this SHOCKING photographic evidence of a new species of giants spawned by the red chert of our beloved mother hill.

But, much like redwood trees, even the most massive Bernal giant starts out small:

Already she knows that she is Person of the Chert. Here we see her making an offering of chert to her Bernal forbearers.

Keep an eye this creature… one day, she will be huge.

PHOTOS: From top, Telstar Logistics, @willak, @wendyverse

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

Our Seismologist Explains Why the 1906 Earthquake Did Little Damage to Bernalwood

As we noted earlier, today is the 105th anniversary of the Great Earthquake of 1906 — the infamous M7.8 rupture along the San Andreas Fault that severely damaged San Francisco before subsequent fires did the rest of the work destroying much of the City. The devastation was near-complete in the core of San Francisco, but Bernal Heights rode out the disaster relatively unscathed.

There were several reasons for this. Geologically speaking, Bernalwood is actually closer to the San Andreas than downtown, but the solid chert bedrock that makes up Bernal Hill didn’t shake nearly as hard as the soft sediment and artificial fill of the Financial District, Mission, or SoMa. That same chert explains why Bernal residents often miss smaller quakes that rattle people in other parts of the City.

That said, geology was secondary to Bernalwood’s survival in 1906. At the time, Bernal Heights was very much a part of the relatively-unsettled outskirts of town. There were fewer structures in Bernal to be destroyed, and most of the buildings that did exist were wood-framed working-class homes. Even on bad soil — but especially on chert! –wood structures perform better than masonry in strong shaking.

Though it was relatively uninvolved in the destruction, Bernal Heights played a big part in the phoenix-like rebirth of San Francisco in the years immediately following 1906. Amid the transition from the tent camps and wooden shacks that occupied places like Dolores and Precita Parks, people noticed that Bernal had largely escaped the catastrophe, and that it might be a (somewhat) safer place to be during any future earthquakes. Happily, that’s still true today.

PHOTOS: Top, Bernal Hill chert, by Telstar Logistics. Below, earthquake shacks in Precita Park, 1906, via Bernal History Project.