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  • From: EarthNSky <erthnsky AT bellsouth.net>
  • To: homestead AT lists.ibiblio.org
  • Subject: Re: [Homestead] Pretty, and pretty scary, California map
  • Date: Sun, 20 Apr 2008 11:19:46 -0400

It is the type of fault. I believe that before Northridge, we did not
know much about blind thrust faults. They had to create a way to find
them, and they are still finding them, most disturbingly under largely
populated areas-usually valleys. I got the following off Wikipedia, which has a good intro page on Blind Thrust Faults:

If the region is under active compression, these faults are
constantly rupturing, but any given valley might only experience a
large earthquake every few hundred years. Although usually only a
Magnitude 6 or 7 earthquake, it is especially deadly because the
seismic waves are highly directed, and the soft basin soil of the
valley can amplify the ground motions by over a factor of 10.

That soft basin soil is also subject to liquifaction in some cases, and we saw that at Northridge and Kobe, both blind thrust quakes.
The acceleration rate is because it is literally under foot and the waves are compacted and funneled, like a tsunami as it nears shore.

I probably shouldn't have picked the Northridge quake in my 'imagine' post because that kind of quake doesn't get much bigger than a 7, but for a lot of folks in SoCal, that was 'the big one', at least the biggest one they have experienced, *and* that was the one that Gene had mentioned.

Bev




Lynda wrote:
Northridge was different and is "special" in its own right. It had
the highest acceleration rate ever recorded. It was the actual 6.7
that was the culprit in that instance.


--
BevanRon of EarthNSky Farm NW Georgia USDA Zone 7
34.498N 85.076W Bev is Earth, Ron is Sky





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