Geology

Earthquake Rattles Bernal Heights; Chert Defenses Hold, Matt Nathanson Predicted It

/ Todd_Lappin / 15 Comments

earthquale17aug2015

So, didyafeelit?

We definitely felt it in the Bernalwood Action Newsbedroom. It was a quick jolt, and the house creaked a little bit, and then it was over. The US Geological Survey says it was a magnitude 4.0, centered near Piedmont.

seismicevent.

Once again, we are reminded to give thanks for our blessed Bernal Heights chert:

chertislove2

Although, things still got pretty gnarly at Neighbor Arno’s house:

Meanwhile, you may recall that the weather last weekend was rather conspicuously hot. And the air was rather conspicuously still.  And then this morning, the ground shook. Coincidence? Well, let’s just say that neighbohood rockstar and accidental seismologist Matt Nathanson wasn’t surprised:

IMAGE: Top, Seismic shake map from the 4.0 earthquake, 14 August, 2015.

New Map Reveals the Lost Waterways of Bernal Heights

/ Todd_Lappin / 5 Comments

seepcity.bernaldetail

In his amazing new Seep City map of San Francisco’s lost creeks, springs and waterways, natural history researcher Joel Pomerantz reveals the places where groundwater once flowed in Bernal Heights.

Here’s the story it tells:

[On the map] today’s land forms are shown with 5-foot-interval contour lines. At this level of detail, we can easily see where human activity has filled extensive portions of the bay and where streets, highways, reservoirs and railroad grades cut into hills.

Our city had significantly more water before it was developed. Consequently, most of the water shown is from historical sources. The purple squiggles are bedrock springs found today. Natural and artificial lakes present today are outlined in white. Creeks of today are highlighted yellow.

Only a couple creeks still flow on the surface today. Finding them can be a challenge without this map. Some are virtually unknown.

The detail is remarkable. Here’s a close crop of northeast Bernal, with Precita Creek running along the upper part of the map and draining into the intricate Islais Creek watershed (where Bayshore stands today). Notice also the two active springs on the northern slope of Bernal Hill:

Bernal.SeepCity.bwood

And when you pull back to look at the city as a whole, you see how Bernal fits in to a much larger ecosystem:

SeepCitymap.bernalwood

Want a copy of Joel’s maptastic creation? Visit his Kickstarter page, where you can order a map in your favorite size.

IMAGES: Courtesy of Joel Pomerantz

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

/ Todd_Lappin / 24 Comments

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

/ Todd_Lappin / 2 Comments

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

/ Julian Lozos / 27 Comments

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

/ Julian Lozos / 7 Comments

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.

Meanwhile, Will Bernalwood Get Hit By a Radioactive Cloud?

/ Todd_Lappin


It goes without saying that the situation in Japan is upsetting. As fellow fault-dwellers, most of us feel a spontaneous sympathy toward others who endure the calamity and loss of an earthquake — and a 9.0 is a VERY big quake.

Then there was the tsunami. Luckily, that’s not too much of a danger for us, because a) Unlike Japan, most of California’s faults are located onshore, and b) We live on a hill that’s shielded from the ocean by an even bigger hill.

But Japan’s crisis may yet arrive on our front doors. Thanks to the out-of-control nuclear power plant at Fukushima (which, it should be remembered, was crippled by the tsunami, and not by the earthquake) it’s entirely possible that the disaster could reach us here — in the form of a radioactive cloud.

The experts say we probably don’t have too much to fear. But if you want to monitor the situation from the safety and comfort of your own fallout shelter, visit a special site created by the Central Institute for Meteorology and Geodynamics in Austria. They’ve built a series of animated maps that model the dispersion of the radioactive cloud. So far, we sit just beyond the nuke cloud’s reach:

But if things get even nastier at Fukushima, Bernalwood residents might want to check these maps regularly to know when it’s time to take precautions.

Visualize the Topography of Bernal Heights in 1853 (Hint: More Wet! Even More Hills!)

/ Todd_Lappin / 8 Comments

Although no one is alive today who ever once saw it, much of the land around Bernal Hill used to be riverbeds and wetlands — particularly to the north and east. But what exactly was the local topography like roughly around the time of the Gold Rush, before all the infill and reclamation that made yesterday’s wet parts dry today?

Architect Glenn Lym has created a 3D CAD map that illustrates the answer. Combining topographical data with historic surveys and a 2010 street grid, Glenn’s way-cool map reveals what was where around Bernalwood in 1853. Glenn explains:

The pics show the 1852-3 US Coast Survey showing Bernal, the Mission and Potrero Hill as they were, as if overlain by the current shoreline and the current streets (101 and 280 shown in orange). Among the items here are:

1. The old Precita Creek Marsh that was a part of Islais Creek and Marsh sneaks up what is now Cesar Chavez, the creek itself shown wiggling between Chavez and Precita Streets on the Bernal side of Cesar Chavez.  Note that Precita Street zigzags parallel to the old Serpentine Road/wall that was erected in the 1800’s, with the Precita Creek running down in the valley between these two landmarks (EDITOR’S NOTE: This is why Precita Street zig zags, even today.):

2. Bernal Heights had two other major peaks to it, to to the north east of the current peaks – roughly under what is now the flat planes that lie between Peralta, Rutledge and Franconia Streets .  Vicky Walker of the Bernal History Project sent me a couple of their aerial survey maps that show that these two peaks were removed sometime between 1938 and 1948.  Terry Milne said that they have been trying to find records which usually exist for 1900’s large excavations, about where all that hillside was dumped, but so far to no avail.  Note that the peak between Rutledge, Massasoit and Brewster was not just chopped off, but gouged out from the Bernal hillside:

Lots more detail on Glenn’s clever 3D CAD project here.

Images: Courtesy of Glenn Lym

New Earthquake Zoning Maps Should Not Rattle Bernal Heights

/ Julian Lozos / 6 Comments
Image snapped and modified from California Geological Survey website

Uh oh?

Last week, the California Geological Survey released its latest set of fault zoning maps. These divide the state into a rectangular grid, with the parts of the grid containing active faults marked in red. Unsurprisingly, there’s a lot of red throughout the state, particularly around the Bay Area and greater Los Angeles. Bernalwood happens to fall along the edge of one of those squares.

So, what does that mean for us? Is the ground beneath Bernal more stable now, or less?

Luckily, the new maps mean very little for us everyday residents. Living within a red square of the grid doesn’t mean the entire square is riddled with active faults that nobody knew about until very recently. It just means that, somewhere within that square, there’s at least one active fault that can cause some mayhem. In the case of the square that contains Bernal Heights, that fault is the San Andreas. The shortest distance from the top of Bernal Hill itself to any part of the San Andreas is still a good 6.25 miles.

These new maps were created primarily for the sake of real estate developers. They’re part of the Alquist-Priolo Zoning Act, which was enacted in 1972 in the wake of the 1971 M6.6 San Fernando earthquake. Aside from some strong shaking that knocked down buildings and freeway overpasses, one of the major problems with that earthquake was that strands of surface faulting popped up in people’s houses unexpectedly.

The initial Alquist-Priolo criterion was that commercial structures or large-tract housing developments may not be built within 50 feet of an active fault, to avoid the possibility that half of a house may become offset from the other half by 20 feet or so. It’s difficult to avoid strong ground motions in a fault-ridden place as California, and a 50 foot distance from the fault isn’t really going to help much in terms of shaking, but avoiding structural surface ruptures is basically as simple as knowing where the faults are.

So, what these new maps (and the older ones) mean for developers? Anyone who wants to build within one of the red squares on the grid must now consult a more specific fault map to determine where they can actually build.

And that brings us back to the question of what all this means for those of us in Bernal Heights, the rest of San Francisco, and California in general. It means — as we already knew — we live in earthquake country. The map doesn’t say anything about shaking hazard, given that closer proximity to the fault generally means higher ground motion. For that there are separate maps for potential ground motion, and they all put San Francisco in a bad place.

But within that, Bernal’s solid foundation of chert means we’ll shake less than the unconsolidated fill in places like the Marina or SoMa — even though we’re in a red box and they are not. So we’ve got that going for us