Wednesday, September 10, 2014

Napa earthquake strong ground motion

Remember, this is the earthquake that I caused by visiting the Bay Area for a day.  :)  It was an M6 strike-slip earthquake, which shouldn't cause any damage in California, but it did, simply because there were a lot of weak buildings still standing.  Most of the rest of the state has had all the weak stuff knocked down.

It also showed that all the retrofit for historic brick buildings isn't worth a shoot.  This was also shown in Christchurch.  Unlike that earthquake, we have a lot of strong ground motion records.


Soft soil seems to show 50-100 cm/s, which is enough to knock down buildings.  Nearby recordings on firm ground showed 5 cm/s, which is strongly felt and can knock things off buildings.  People get alarmed at about a fraction of a cm/s, so there is a big span.

Well-built structures should shrug off 100 cm/s, and I really believe that all structures should be built to ride out this level of shaking.  Sadly, this is not the case.

To derail a train, or to do anything serious to 90% of office buildings, you need 100 cm/s, which you only get within 5 km of a fault on soft ground.  That makes all this talk of an early warning system a Grand Stupidity, much like the Bruce Deep Thing.  That doesn't mean it won't get built.  Over the years, I have accommodated myself to the concept that Grand Stupidities are a necessary part of the human condition.  :)

Early warning systems only work at 5 cm/s, and if you are sensitive to that, then you got bigger problems than a distant earthquake.

Additional:  Actually, we all need an "Anti-Warning System", since people panic and leave a building at 0.1 cm/s.  The PA would go:  "We are about to experience mild earthquake shaking.  Please sit down, secure your coffee, and enjoy the ride."

Add2:  Nice article on wine barrels.

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