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.