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Paso Robles and other earthquakes 1

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The original message was too long, so I divided it in two, and removed the attachment...
 
Part I: Happy Holidays!
 
I just came from the Paso Robles Area where I was inspecting damage after the recent earthquake.  Here are some observations from the site.
 
Several newer - and quite expensive - houses sustained substantial cosmetic and even some structural damage which I would not expect from such structures.  Generally it proved to me that drywall and stucco shear walls should not be used - especially in irregular houses - no matter how low the calculated forces/stresses are.  
 
I also observed distinctive soil damage that I did not see a lot during Northridge.  In one instance, a three-car garage, separated from the main house by a breezeway, visibly settled more than 1 inch (without any substantial damage/distortions inside it!).  The garage was obviously built on fill while the house proper had a sub-grade level with retaining walls.  Strangely, the house is about 15 years old, and never had settlement problems (no noteworthy old cracks in stucco).
 
The other case was even more amazing - a 3000 sq. ft one-story house on a flat lot on top of a hill (as many of them are in that beautiful country, imagine the view).  The house sustained very little - barely any - damage to the superstructure, in spite of heavy tile roof, brittle finishes, and irregular shape/composition.  But the slab it was built on cracked intensively - long linear cracks in, and under, ceramic floor tiles.  Some of the cracks have vertical offsets up to 1/2." 
 
There was some evidence of topsoil fissuring and distortion/shifting of the underground pipes; a large swimming pool had drained since the earthquake (in 7 days). 
 
It looks as if the apparent underlying rock-like shale was shifting along its weak planes...  Can it happen that way? And if so, why so little damage to the superstructure?    
 
To illustrate the force of that earthquake: a crew-cab F150 was parked in the garage (naturally, a 10-car garage).  The sectional door is now bent and would not open - apparently, because the truck was slammed against it from inside, and then was pushed back (~36" amplitude movement).  All that is verifiable by clearly visible tire marks on the garage floor.   
 
Also, it was nice to see the tower in the downtown Paso Robles already replaced with a wood-framed OCB-sheathed one.  BTW, a very neat job, and, obviously, much safer structure (attached)... I presume it is the one that collapsed and was in all news.
 
Other than that -
 
Happy, Healthy, Peaceful, and Prosperous New year to all. 
 
Steve Gordin, SE
Irvine CA