The topic of this set of posts arises from the terrible wildfire season we’re experiencing in 2020, on top of the terrible pandemic, on top of the unemployment crisis, and other more distant events that may be affecting our friends and family. We’re all learning a lot about these topics because directly or indirectly they personally affect each one of us.
When we think about them, we each come from our own place, and my place is geology. I find myself thinking about the ways these new disastrous events feel compared to the old familiar geological risks that have been on my mind all along.
So I thought I’d talk about earthquakes and their risks more deeply and more frankly than I usually do on this blog. I’m doing this because the catastrophes of 2020 have given us all a lot to think about, and if your perspective on those things has evolved — for instance, the way you react to masks — maybe you can think about earthquakes from a different perspective too. Let me stipulate right off that this is only my earthquake advice, and that I speak for no one else and do not pretend to supplant the US Geological Survey, the state Office of Emergency Services or any other authority, although I endorse everything they say.
Oakland is a special case when it comes to earthquakes. Here the risks are different from most other cities because the Hayward fault runs through the whole city, including some important parts. And the long-anticipated large earthquakes — not just one Big One like the 1868 earthquake but about ten times as many damaging Pretty Big Ones — will hit the whole city hard, not just along the fault.
Every so often, someone writes to me with a question about buying or living at a homesite on the Hayward fault. For them it’s always about the fault itself. Something about a rip in the ground on a million-dollar piece of property brings out these sharply felt concerns about “The Fault.”
I’m not a licensed geologist, so what I can say is legally limited. I always say that first. Practicing geology in California without a California geologist’s license is a crime. If someone really wants specifics, they need to engage the correct professional — architects, contractors, geotechnical engineers, home inspectors, lawyers — and all of those experts are legally limited too. The expertise of specialists is partial and does not add up to certainty or wisdom.
The ways I try to help questioners are to provide data about the hazard and correct errors in their thinking about the risk. Here’s an example: a person wrote me with a question about a house that was apparently right on the fault. They’d looked up all the maps they could find, but wanted newer maps, maybe unpublished maps, that showed the fault’s location more precisely. They were asking for more knowledge about the hazard, namely a rip in the ground that damages the house.
Before answering, I had to step back, because better maps aren’t available (if they even exist) and aren’t necessary for making a decision. The thing about the fault is that we don’t precisely know where the rip in the ground is, except in a few well-defined places. And even where we do, the fault is not just a sharp line on the ground but a zone, sometimes tens of meters wide, that will warp and crumble when the fault gives way. In other places, like the south end of Redwood Heights, we aren’t sure where it even is.
Earthquakes happen deep underground, where most of the energy is, and the surface where we live is near the outer edge of most quakes. Up here along that outer boundary, being “on the fault” gets fuzzy. Every large earthquake is different. You can’t count on the ground breaking in the same place each time.
The state’s official solution to this uncertainty is the Alquist-Priolo Act, under which the authorities do their best to map active faults and then establish a wider zone around the faults (usually 50 feet) that effectively has the same hazard. Insurance companies, for instance, rely on these. So my answer to questions about better fault maps is that (1) this is as good as they get and (2) this is as good as they need to be.
There is one way to get more certainty, which is to look at the ground very carefully, and call in an expert to confirm any suspicions that arise. That’s because the Hayward fault doesn’t only rip the ground; it also pulls the ground, very slowly, all the time — the process called aseismic creep, or just “creep” for short. If creep on the fault is already pulling the house apart, the signs may be there, although deep landslides can create the same signs.
This is about the best one can do in gauging the hazard for a particular site. As for the risk, well, being in an Alquist-Priolo zone presents a relatively high risk and being directly affected by creep presents a risk that is absolutely high, not just relatively high. But like I said, the next rip in the ground may come somewhere else, maybe across the street, so the risk is still not 100 percent certainty.
This is where I step even further back, back to where knowledge may edge into perspective. From here, all natural threats are alike at their core. I can tell you this: Earthquakes are inevitable threats, but homebuyers roll the dice and most of them come out winners, because they survive without experiencing a Big One and their houses sell at a profit, and in the meantime they have enjoyed years of pleasurable life in their homes. Even a house directly on the fault, if it’s well sited and well made, will not kill you in a major earthquake.
I think that’s the attitude most people around here have, a California attitude. It strikes outsiders as odd, as I recorded in a story at the end of this post from 2017.
But life and death are not the only risk considerations. There’s also injury, damage and inconvenience, matters that mix concern with money. I’ll get into that topic in posts to come.
Oakland Geology 
31 August 2020 at 3:57 pm
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