Welcome back Jax! The E’ville Eye’s first contributor is taking a break from writing her first novel … and getting back to some bloggin’! J.R. Hampton takes on something that is generally on the back of the minds of most E’villains (and Californians for that matter): Earthquakes & Liquefaction (Now would be a good time to assemble that go bag & emergency plan that you’ve been putting off).
Feature Image: Which side off the liquefaction map are you on (Hint: The railway is built on the orange side)?
The Third Uniform California Earthquake Rupture Forecast
Living in California, we don’t have to deal with tornados or hurricanes or blizzards: that stuff happens to other people in other places. But earthquakes happen to us. And the most recent Third Uniform California Earthquake Rupture Forecast (UCERF3) is the latest word on what that means for people who live and/or work in Emeryville.
UCERF3 tells us that we can expect to experience an earthquake magnitude ~6.7 (Loma Prieta was 6.9) about once every 6.3 years. Improvements in earthquake science fine-tuned the forecast in our favor, and this estimate is down from the previous report’s estimate of an earthquake that size every 4.8 years. But UCERF3 also predicts that the chances of a big one, an earthquake of magnitude 8.0 or larger has grown from 4.7% to 7.0%.
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The effects of the 2011 Japanese Tōhoku earthquake and resulting tsunami could be felt all the way to Emeryville
Let me explain ‘the big one’ in terms that anyone who was living in the Bay Area on October 17, 1989 will understand. A magnitude 8.0 earthquake would be the equivalent of 30 Loma Prieta earthquakes happening in the same place at the same time. The Richter scale is a base-10 logarithmic scale, so you might ask, why 30 Loma Prietas and not 10? It’s because an earthquake that registers 8.0 on the Richter scale has a shaking amplitude (the “size of the wiggles” on a seismograph) 10 times that of an earthquake that registers 7.0. But the energy released by the magnitude 8.0 earthquake is 31.6 times as much as that of the 7.0.
Whether we are in for a moderate quake or ‘the big one’, the most likely suspect fault-wise is in Emeryville’s backyard. The probability of an earthquake along the Hayward Fault with a 6.7 magnitude or greater increased three-fold between the last UCERF report and this most recent one. Why? Because there hasn’t been a lot of activity on that fault and when that happens, tension builds.
“At this point, the Hayward Fault is pretty much reloaded,” said Menlo Park-based USGS earthquake scientist Wayne Thatcher in an interview with Lisa M. Krieger, San Jose Mercury News.
My first moderate earthquake was a 6.7 with an epicenter about 50 miles away. I woke up to a noise that sounded like a freight train heading for my bedroom window. The next thing I remember is my Dad in the hall telling my sibs and me “not to worry … it’s just an earthquake”. My father, always one for a good disaster, later reported that our town hadn’t suffered much damage. “Except near the harbor,” he said. “All that landfill. The stuff is like quicksand in an earthquake. A couple of buildings sank a little. Liquefaction.”
Emeryville can expect a decent helping of that along with whatever else the Hayward Fault dishes up in our future. Approximately 73% of the area in red—a large portion of Emeryville, is predicted to liquefy in the event of a magnitude 7.1 earthquake. For the purpose of orientation, the edge of the red area is about where the railroad runs through Emeryville.
Liquefaction is when soil that appears solid loses strength, and instead of behaving like a solid, behaves like a liquid. Acts like quicksand. Not all soils will liquefy. But that red area in Emeryville? It’s what geologists call AFEM (artificial fill on estuarine mud)—exactly the type of water-saturated soil (and fill) that is susceptible to liquefaction.
During an earthquake, according to Earthquake Engineering New Zealand March 2011,“ … the shaking is so rapid and violent that the sand and silt grains [composing the soil] try to compress the spaces filled with water, but the water pushes back and pressure builds up until the grains ‘float’ in the water. Once that happens the soil loses its strength—it has liquefied.”
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Think of it this way. If you were to strike the side of a bowl filled with a dense material like soft cement, you wouldn’t see much in the way of waves on the surface of the bowl. That’s because the energy from your strike would travel through the cement as a high frequency wave with low amplitude. This is how seismic waves move through bedrock. Take a look at the Yerba Buena ‘bedrock’ seismogram on the far left in the animation above. Fill the same bowl with water and hit it, and you’ll generate a wave of high amplitude and low frequency with waves that can be clearly seen on the surface of the bowl—like the Treasure Island seismogram on the far right.
Buildings don’t float. When soil liquefies during an earthquake, it is no longer able to support the weight of whatever is lying above it—a building, a road or even dry soil.
And so like Humpty Dumpty, it all falls down. At least some of it will in Emeryville, with a 73% probability.
For my father, Edwin Earl Hampton, who left earthquake country on October 17th, 2003—fourteen years to the day after the Loma Prieta earthquake.
Further Reading & Resources:
Uniform California Earthquake Rupture Forecast, Version 3 Report [PDF]
Big Bay Area quake: When and where is it most likely to happen? | SJ Mercury News
Quake maps show shaky East Bay soil / Risk of ‘liquefaction’ in north Alameda County | SFGate
Landfill and Liquefaction on KQED’s Quest
KQED Audio Segment: New Forecast Increases Odds for Huge Earthquake in California: