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If haven’t read this month’s New Yorker article about the Cascade fault that spans from Vancouver down to Northern California (http://www.newyorker.com/magazine/2015/07/20/the-really-big-one), you definitely should.  It’s a fascinating read and has a lot of useful information, especially in the light of a little jolt that we had this morning.  Here are some take aways from the article and a little bit from myself.   While this may not be so useful to the one’s safety plan, but it should help raise our community’s EQ IQ:

  1. If the earthquake of the 1989 Loma Prieta magnitude would have hit where this morning quake struck, the shaking would have felt approximately 12 times stronger. Because it was 70 miles away, the predominant natural frequencies of the earthquake vibrations affected to the greater extent taller and more flexible structures, like taller buildings, freeways and bridges. The houses and other more rigid structures were largely unaffected.
  2. As you may have noticed, the jolt was quite hard and shook people’s houses a bit as its proximity and dynamic characteristics affect houses (short, stout structures) a lot more than shakers on other nearby faults including the San Adreas. Depending on where the earthquake epicenter is and how long of the fault’s stretch ruptures, the effects  and damage may be quite different than what people remember from 1989.
  3. The Richter scale that earthquakes strength is measured with is a base-10 logarithmic scale. From the wikepedia article on the subject: “As measured with a seismometer, an earthquake that registers 5.0 on the Richter scale has a shaking amplitude 10 times that of an earthquake that registered 4.0, and thus corresponds to a release of energy 31.6 times that released by the lesser earthquake.”
  4. There is a pretty good correlation between the time that the shaking goes on and the earthquake’s magnitude. The 1989 earthquake in Loma Prieta, California lasted about fifteen seconds and had a magnitude of 6.9. From the NY article: “A thirty-second earthquake generally has a magnitude in the mid-sevens. A minute-long quake is in the high sevens, a two-minute quake has entered the eights, and a three-minute quake is in the high eights. By four minutes, an earthquake has hit magnitude 9.0.”
  5. Don’t panic, the Hayward fault can produce a maximum credible strength eq of “only” 7.5, while San Andreas of 8.2, which a fraction of the 9.0 to 9.5 that Chile, Japan and NW coast has experienced. Be thankful that you don’t live on the coastal area of Washington or Oregon and get ready.
  6. Bolt your house; seismically retrofit your crawl space, secure the walls that have large openings and you will most likely be OK. Refer to the website for Association of Bay Area Governments for a free DIY set of plans that was developed together with the Structural Engineers Association of NorCal for residential crawl space strengthening. http://resilience.abag.ca.gov/residents/planset/. Obviously, if you can afford more or if you are considering a remodel, talk to a Structural Engineer.
  7. If you live in the old apartment building with 5 units or more and 3 stories or more, the city of San Francisco and Berkeley have passed mandatory ordinances to identify buildings that have Soft or Weak ground floors, mostly due garage door openings or tuck under parking and with units above. The identified properties are required to be retrofitted within the next few years. Refer to our earlier posts on that topic.  The city of Oakland is in the process of adopting an ordinance to have the owner to investigate apartment buildings that meet that criteria.  For soft story buildings, the owners will be required to seismically retrofit them.  Some of these retrofit costs will be allowed to be passed on to the renters.
  8. I hope that we all get spared the devastating effects of a large closeby earthquake. But we all, for one reason or another, chose to live in SF Bay Area. The least that we can do is to get ready and have an action plan.  In the meantime, enjoy our beautiful summer and Indian Fall weather, I heard that El Nino is coming (fingers crossed).

Yours truly,

Sasha Itsekson, Structural Engineer

Not an Earthquake Scientist or a Weather man

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