PSN-L Email List Message

Subject: Re: dueling earthquakes
From: "Thomas Dick" dickthomas01@.............
Date: Tue, 29 Apr 2008 12:15:08 -0500

To bring a "possible" California similarity into this, there have been 
similar patterns within the San Francisco Bay Area. Yes, we have had quake 
swarms all over the bay area, particularly in one area in what's known as 
the East Bay (Concord, Alamo area). The swarm lasted for a few weeks with 
the largest reaching 4.5 and then the activity abruptly stopped.

National TV News a few weeks ago suggested a major quake out there was to be 
expected. I think that thinking was stimulated by the recent quake activity 
in the Alaskan chain and down in South America..

But the swarms in Nevada do seem to reflect the dueling quake activity that 
occurred in our area during the 1800's (I wasn't here then) when there was a 
M6 quake in the east bay, followed years later by M7 quake in the Santa Cruz 
Mtns area (Loma Prieta area), only to be later followed by a M7 back in the 
east bay. It was sort of a battle of the bay - east bay vs. south bay. The 
area spans about 150 - 200 miles.

The area out there is so complex.

I've been watching the depth of many of the quakes and most of them seem to 
be in the 0-7km range but there is the consistent number of events occurring 
deeper, in the 10-18km range too. Those deeper events might be more 
indicative of something maybe unusually large.

Any comments?
Quakes in southern Illinois are deep because most of the faults are very 
old. One "felt" quake in 20 years is the norm. Most quakes the do not last 
long -- maybe less than 20 seconds --several of these recent quakes went on 
for over twice that time frame ....and usually are not centered in one area 
like this sequence. The sediments are also deep -- as much as 300 ft in many 
areas. This means they are felt! The shaking extends out. The news didn't 
really cover the aftereffects because there were no human injuries. I talked 
to an acquaintence living in the six mile diameter area of these quakes. He 
is a farmer. His home and outbuildings did just fine. But his mother's home 
was seriously damaged and he didn't know if it was worth fixing. And there 
are others in that area too. I expect things to quiet down here but Nevada 
and west coast may not be done.


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