Did You Feel That?

It hasn’t gotten much play in the media, but there was an earthquake here in the San Francisco Bay Area early Sunday morning.

Ah, who am I trying to kid? The papers, TV, and Internet have been full of it. And full of stories about the earthquake, too. That being the case, I’ll skip the detailed retrospective; if you did somehow miss the news, there are plenty of places for you to catch up.

I will note that there have been almost a dozen aftershocks of magnitude 2.5 or higher, including a 3.9 at 5:33 this morning. I slept through it, but it woke Kokoro up, and she in turn woke me up when she jumped off the bed. That meant I was awake at 5:35 to feel a 2.7 quake (it shook the bed slightly more than ‘Nuki did when he jumped on a few minutes later). There was also a 3.0 aftershock at 6:45, just enough to make my monitor sway a little.

Just to be clear, we’re about 15 miles from the center of the original Sunday quake, and most of the aftershocks have been even further away. Despite the way it looks on the USGS map, the aftershocks are not moving north. Presumably whatever Mother Nature is annoyed about is in the Napa/American Canyon area, and she’ll continue to poke at it until she’s satisfied.

But you’re not here for my uneducated speculation about vengeful Earth deities. I know you’re actually here to find out whether the Bay Bridge is still standing.

In short, yes. None of the major bridges in the Bay Area suffered any damage. But don’t get too excited. That doesn’t mean we can relax and accept Caltrans’ assurances that the bridge is perfectly safe.

While people and structures near the epicenter of the quake–i.e. the residents and city of Napa–felt a level IX shake intensity (the USGS characterizes that as “Violent” perceived shaking with “Heavy” damage potential), near our house, it was down to the lower reaches of level V (“Moderate” shaking and “Very light” damage). Continue south to the bridge and you’re well into level IV (“Light” shaking with “none” potential damage).

The Bay Bridge, in other words, wasn’t particularly tested by this quake.

While the damage in Napa and surrounding communities was significant, it could have been much worse. From what I’ve read, the 6.0 quake was just below the level where wide-spread, major damage with massive casualties and major infrastructure damage is expected. That threshold is somewhere around 6.2. The intensity scale is logarithmic, so a 0.2 difference is much larger than it sounds, but the point is that this quake wasn’t strong enough to be expected to cause bridges to collapse.

Until we have a similarly-sized quake on the Hayward or San Andreas fault, the Bay Bridge cannot be said to have been seriously tested, and we’ll have to continue to rely on the engineers’ estimates. Seismologists say we could have such a quake any time. As of 2008, the USGS estimated the chances of a large earthquake by 2038 at 31% for the Hayward fault and 21% for the San Andreas. Note that by “large,” they mean 6.7. That implies that a 6.0 quake similar to Sunday’s is significantly more likely.

For a great number of reasons, I’d prefer to put off a live test of the Bay Bridge’s seismic stability as long as possible.

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