In Southern California, the San Andreas Fault makes up the plate boundary between the North American plate and the Pacific plate, with that single strike slip fault taking up a majority of the motion between the two plates. Northern California is a bit different. The San Andreas fault runs offshore to the west of San Francisco and it takes up part of the plate motion, but another part of the plate motion is taken up by a strike slip fault to the East, the Hayward Fault.
The Hayward Fault runs through communities to the East of San Francisco Bay, including Oakland and Berkeley, where it runs through the campus of the University of California (https://tmblr.co/Zyv2Js1Q0m6xP). Part of the Hayward fault last ruptured in a major earthquake in 1868, when there was a much smaller population than there is today. The nearby San Andreas fault segments last ruptured in the 1906 Earthquake that devastated San Francisco. Faults often don’t behave well enough to say that they will rupture every (blank) number of years, but it is absolutely certain that this fault will break again, triggering a major earthquake to the east of San Francisco Bay, in an area inhabited by millions.
To understand and prepare for this upcoming disaster, scientists have produced computer models like this one, simulating how the area around the fault will shake given a reasonable rupture along the Hayward Fault. This fault model has been nicknamed the HayWired Scenario. The colors are coded to the Mercalli scale of Earthquake intensity, which characterizes earthquakes based on how strong the shaking is at any particular spot (https://tmblr.co/Zyv2Js2BNeuY7).
You can find interesting details of how the local geology moves the energy of the quake around. Look for the extreme shaking in areas right next to the Bay; these areas are loose, water-saturated sediments and those areas typically amplify seismic shaking. Across the Bay, the Marina district in San Francisco is built on these sorts of loose sediments and it suffered heavy damage in both 1906 and 1989, in part because these sediments undergo liquefaction during quakes.
Aside from liquefaction zones, the shaking is most intense right next to the fault, and then energy is carried away by other major geologic structures. Note the slow moving clock at the upper left – a full rupture of this fault would create an earthquake lasting more than a minute.
Last week, there was a magnitude 5 earthquake off the coast of California. Scientists and engineers have developed systems that can trigger an “early warning” of several seconds, sent to smartphones and other automated devices. In California this system is only preliminary, but it has been deployed elsewhere in the world including Japan and Mexico. During the small quake last week, the early warning system gave a warning of several seconds before the strongest shaking arrived.
Several seconds isn’t a huge amount of time, but it can make a lot of difference. Trains can be slowed down, power stations can be put into safe mode, doors can be opened on fire stations to let trucks out, and people can brace. One more creative example of why only a few seconds warning could be hugely important – one southern California Geologist was apparently in the dentist’s chair at the time the shaking started. That’s not a procedure you want to be undergoing when an earthquake starts, so if your dentist can move away a few seconds in advance…that seems to be worth the cost of the program right there.
The Federal government budgeted several million dollars to begin deployment of this warning system in its 2018 budget a few weeks ago. It won’t do everything, but by this time next year there’s a good chance that smartphones in California will be able to link to this system, giving anyone who downloads the right app a chance at an Earthquake Early Warning
Video credit/HayWired scenario (USGS)
https://wim.usgs.gov/geonarrative/safrr/haywired_vol1/