After the Napa quake a little while ago, I found myself curious. I’ve known for a while that the Hayward Fault in the East Bay stands a decent chance of tearing loose in a big way. If I’m still here when that happens, how bad will it be? What will it feel like for me, on the other side of the Bay? How will that compare to what I felt during the Napa quake? (How frightened am I likely to be?)
If reading about destruction from earthquakes is likely to upset you, don’t go behind the cut.
My curiosity led me to the Mercalli scale, which I hadn’t heard of before. The Richter scale is the one everybody knows; it’s a fairly precise measurement of how much energy was released during a quake. But a 6.0 quake feels very different when you’re sitting on top of the epicenter vs. when you’re fifty miles away. The Mercalli scale is a much more subjective assessment of how bad the shaking is in a given location, and as such, there’s no single number for a particular event. It depends on where you are. Nor does it even vary regularly with distance from the epicenter: local geological conditions can mean that a spot quite nearby suffers less shaking than one much further away.
According to a map I found, my town was roughly at a 4 during the Napa quake. That sounds about right: some people woke up, some didn’t. There was some alarming rattling going on. But nothing broke, and I didn’t hear about any damage to buildings locally.
Okay. So what about the Hayward Fault?
Here’s your answer. Or rather, your answers: there are seven scenarios modeled there, varying in magnitude and epicenter, including one worst-case scenario where Hayward going also sets off the Rodgers Creek Fault to the north. (Fortunately, this one is much less likely.) The animation shows the Mercalli rating in each location via spreading waves of color.
Watching the videos is morbidly fascinating. The Peninsula gets off fairly lightly: in most scenarios the effect where I live doesn’t rise above a 6, and even in the worst case it’s maybe a 7. And a lot depends on where the epicenter is, but not necessarily in the ways you’d expect. San Jose actually fares better when the epicenter is nearby in Fremont than when it’s way off in San Pablo Bay — I presume because the effect proceeds along the line of the fault, which sends most of the energy away from San Jose if the epicenter is nearby. More distant epicenters are worse for Silicon Valley. But really, I’m less worried about them . . . because oh, Jesus, Oakland.
Take another look at the Mercalli scale. Pay attention to the colors. See those videos? See the spreading wave of blood red, shading into black? Those are the levels rated as Intense, Extreme, and Catastrophic. That’s the top of the scale. The worst of it never stays in one place for more than a second or so, but even a second will be enough to wreak enormous amounts of destruction. Yeah, sure, California is prepared for earthquakes — up to a point. Not everything has been retrofitted. And, like the levees in New Orleans, there’s a limit to how much our engineering can take before being overwhelmed. The Oakland Hills are screwed in pretty much any scenario: all those expensive houses are going to come tumbling down. Downtown Oakland will not fare well even under lesser stress. This is not idle rubbernecking for me; I have friends who live there. And even without that personal element, we’re looking at a devastating loss of property and life.
There are some things that can be done. Apart from the building codes that require certain standards on new construction and retrofitting on at least some old construction, there are also early-warning sensors that can help: they don’t give much advance notice, but even one second can be enough time for train drivers to hit the brakes, for safety switches to flip on gas lines so we don’t have uncontrollable fires in the aftermath. Mind you, we’d have to actually install systems like that before they could be useful to us. Which costs money, and that’s a hard political hurdle to clear — even though we know what the consequences will be if we don’t.
Any way you slice it, it’s gonna be bad. The most comfort I can take is in knowing that if I’m at home when it goes off, I’ll be safe compared to the East Bay.