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.

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.

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

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

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