What You Missed When You Missed Glenn Lym’s Talk About the Lost Geology of Bernal Heights

Glenn Lym addressed a full house at the Bernal Heights branch of the San Francisco Public Library on Wednesday night. His presentation focused on how San Francisco transformed the hilly native landscape into flat land suitable for development.

Much of the first half hour recapped Glenn’s HERE5 documentary, which was brilliant. But having first seen that the day before, a second pass helped me understand the process better. Here’s the story:

In 1849, very little of San Francisco was flat. Sand dunes over 100 feet high made land passage impractical between “downtown” and the Mission. Millions of cubic yards of material was moved to create the flat center of San Francisco we see today.

One remarkable photo in the slideshow showed picnickers on a peak of Potrero Hill that Glenn said no longer exists; a spot that is now either Franklin Square or the Safeway shopping center (previously the site of Seals Stadium). I think this may have been called Irish Hill, but I’m not sure. (John Blackburn corrects me in the comments; Irish Hill was on the East side of Potrero.)

In the second half, Glenn showed Coast Survey-based CAD reconstructions of the lost peaks of Bernal Heights, though he wasn’t sure when they had been removed.

Harrison Ryker’s 1938 photos showed a peak at the top of Ripley Street, above the intersection with Peralta, which was missing on a later photo:

An older gentleman in the back, attested by others to have lived on Ripley, said the hilltop removal began in 1939, stopped during the war, and resumed afterwards — leaving the block between Peralta, Esmeralda, Franconia and Samoset flat by around 1950. The debris was probably used to fill Isais Creek, with some of it possibly used as ship ballast.

The fourth peak, where the Franconia/Brewster public gardens are today, south of Rutledge, was removed prior to 1938. Some industry, possibly hilltop-removal, was visible in an aerial photo that showed the Maxwell advertisement atop Bernal Hill, which suggests it happened in the mid 1920s.

Glenn referred to historical posts by Burrito Justice and Bernalwood several times in his presentation, with special attention paid to Burrito Justice’s posts on the Valencia Hotel collapse and Serpentinia, and Bernalwood’s epic post on the history of Army Street/Cesar Chavez’s awfulness.

Bernal’s superior seismic safety was discussed in the Q&A after the talk, though I don’t think our chert was specifically credited.

… In Which I Am Revealed to Be Only Marginally Prepared for a Very Big Earthquake

A few months ago, I volunteered myself, my daughter, and my home to serve as on-camera crash-test dummies for a series of earthquake-preparedness videos produced by Totally Unprepared, a public-awareness organization which describes itself as…

… what happens when you put forward-thinking state agencies, earthquake geeks, social media nerds, a web analytics genius, a professional filmmaker, a hot firefighter or two, and a bunch of unsuspecting Californians in a blender and hit frappe.

During their visit to our home, Totally Unprepared pretty much put us through a blender and hit frappe. But that’s what we’d signed up for, to foster better earthquake preparedness in California — and the Dominion of Bernalwood.

The videos have been now released as a series of installments optimized for Web-length attention spans, and they feature both me and Bernalwood’s brave Cub Reporter. In the first episode, our home is given a thorough inspection, and we are subjected to a somewhat terrifying jostle in an earthquake simulator — which the Cub Reporter endured with true native-Californian aplomb:

In the second installment, we hone our duck-and-cover technique in various awkward and uncomfortable places throughout our home:

The third episode reveals (somewhat embarrassingly) that I had neglected to properly secure the bookcase that sits next to the Cub Reporter’s cute little Hello Kitty bed. DOH!

Thus, with my humiliation complete, I now encourage you to find out more about how to prepare for the Big One.

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 Evaluates Last Night’s Earthquake

So, did you feel last night’s earthquake? I sure did. Heard it too. Good times.

Julian Lozos, Bernalwood’s Senior Seismologist, wasn’t here for the shake, but after it he tweeted up a storm from his high-tech monitoring post. Here’s some of his seismo-punditry:

Yaaaaay, chert!

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.

The 1906 Earthquake, as Experienced from Bernal Heights

Today is the 105th anniversary of the Great San Francisco Earthquake of 1906. So what was the big event like here in Bernal Heights?

Rather uneventful, actually. Bernal itself suffered relatively little damage — thank you, chert! — and the fires that ravaged much of the rest of the City never made it this far south. (As you can see above, Bernal Hill proved an excellent vantage point for gawking the disaster as it unfolded.) Later, parts of the neighborhood — most famously Precita Park — were used as encampments to shelter displaced City residents, which in part explains why the quake ultimately turned out to be a boon to Bernal Heights development.

To put all this in perspective, enjoy this video produced by Vicky Walker of the Bernal History Project. It tells the tale of what the Great Quake was like here in Bernal Heights:

PHOTO: via Bernal History Project

Meanwhile, Will Bernalwood Get Hit By a Radioactive Cloud?

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.

New Earthquake Zoning Maps Should Not Rattle Bernal Heights

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

Our Seismologist Assesses Last Friday’s Earthquake, Bernal Bedrock, and Our Good Fortune

Julian Lozos, Bernalwood’s Seismologist-in-Residence, has been pouring over the data on the earthquake that rocked the Bay Area last Friday afternoon. Here is his report:

Some of you may have felt the magnitude 4.1 earthquake that rocked the Bay Area on Friday evening. Of course, some of you may not have felt it. The solid bedrock of Bernal Hill diminishes a lot of ground motion, so Bernalites may not feel some of the jolts and tremors that put other parts of the City on edge.

Regardless of whether or not you felt it, please take two minutes to fill out the USGS’ Did You Feel It questionnaire about this earthquake. Reports on who didn’t feel anything in an area where plenty of people did feel it are actually incredibly useful to scientists. Reports of earthquake intensity, from extreme to zero, can help highlight underlying the geology of the area. Retroactive shakemaps from Loma Prieta or 1906 show the strongest intensity in areas of artificial fill, and the weakest on solid bedrock. (ie: Bad for the Marina, good for Bernalwood.) So please fill this out! Filing a report for the smaller quakes is good practice for when a bigger one inevitably happens.

Friday’s earthquake was centered near San Jose. Relocations of the waveforms put the quake on the Calaveras Fault, at a depth of 7 km (which is a moderate depth for this kind of fault). This particular segment of the Calaveras is near where the Hayward Fault branches off from it; Some recent studies even suggest that the two faults are connected at depth. The 2007 Alum Rock earthquake — a M5.6 — wasn’t too far to the northwest of today’s event. The southern Calaveras Fault was also responsible for the M6.2 1984 Morgan Hill earthquake, which was widely felt over the entire Bay Area and beyond.

Lest you think that the only thing this earthquake has to do with Bernal Heights was to unsettle a couple of people who aren’t fortunate enough to have our solid foundation, an interesting tip-off came on Twitter shortly after the quake: apparently the land that contains the epicenter was once owned by a certain Jose de Jesus Bernal — the original Mexican landowner who gave our hill its name.

Fortunately, all of his land wasn’t so faulted as this chunk near San Jose. Bernal Hill has some small fault offsets in its chert, but those are not the kind that cause large earthquakes (or even small ones anymore). In other words, we’re solid!

Our Seismologist Reports on Current Trends in Earthquake Science, Predicts Possible Hipster Exodus from The Mission

Julian Lozos, Bernalwood’s Senior Seismologist, has spent much of this week at the American Geophysical Union’s AGU 2010 conference, which is taking place downtown at the Moscone Convention Center. Julian filed this report to update us on the latest scientific thinking about earthquakes, faults, probabilities, and what all that means for helpless mortals living in Bernal Heights:

I attended sessions during which several major debates in earthquake science were discussed. One was about determining probable endpoints of earthquake rupture, and what that means for overall event size. A lot of this is determined by the geometry of the fault trace — which ties in directly to my research! — but regardless of whether or not you believe geometry has a big effect, the whole panel agreed that there are segments of fault in California, even aside from the San Andreas, that are long enough to produce earthquakes up to magnitude 8.

The second debate had to do with whether or not faults have “characteristic earthquakes” — that is, whether or not a fault creates essentially the same event (same epicenter, same length of rupture, same magnitude) over and over. By a characteristic model, a fault would have this large specific event at some recurrence interval, with very few smaller events in the interim.

The opposite end of the characteristic model suggests that faults have a wider range of earthquake sizes, and that the largest events might happen on different parts of the fault from earthquake cycle to earthquake cycle.

Earthquakes can’t be predicted by any means, but faults that don’t behave characteristically are even harder to sort out. In terms of what that means for San Francisco: If the San Andreas is characteristic, we can expect repeats of the 1906 rupture (and maybe some of the smaller Santa Cruz-ish segment ruptures, like Loma Prieta and 1836). A repeat of 1906 wouldn’t be as bad now as it was then, since we have better building codes and disaster response, but it would still be a large-scale disaster. Who knows, maybe more people would move to Bernal Heights after that one as well, after catching on to the benefits of our bedrock? (But would we WANT the potential hipster exodus out of the liquefacted Mission? Ahhhh.)

Anyhow, if the San Andreas is not characteristic, its next big quake in the Bay Area could still be the same general segment as 1906, but with a different epicenter that could lead to rupture directivity that sends even more seismic energy into San Francisco proper than happened in 1906. (In terms of directivity, actually, 1906 was kind of the BEST case. Yikes.) Or, the next big one might be some smaller subset of the 1906 rupture — which was pinned to the south by a part of the fault that doesn’t slip in earthquakes, and to the north by the end of the fault. Yet that’s still big enough to cause a lot of problems.

Of course, the Hayward, the Calaveras, and all the faults to the north of us have similar issues. If the 1868 Hayward event was the characteristic earthquake, well, we’re still in a lot of trouble. But if it was a smaller event within a bigger cycle, the eventual main event could be larger than the ~M6.8 of the 1868 event, and then we’ll REALLY be in trouble — though not as much trouble as the East Bay.

So there you go. Have a nice day!

Photo: The 1906 earthquake as seen from the bedrock safety of Bernal Hill, via the Bernal History Project