BlueHeronDruid wrote: 
When the rock is all broken up or in different bits.  The rocks in the east are good solid things so when they get shook about the vibrations go through nicely.  The rocks in the west are less solid so the vibrations get dissipated quicker.  I think thats right, but let one of the rock geeks give it the say so first.
A bit like hitting a lump of granite with a hammer as opposed to hitting a lump of sandstone. 

 geoff_morphini wrote: 
Okay. I'm stupid. I've never had a cracked wine glass. Try again, por favor? I know rocks. What's a rotten rock?

 NoEnzLefttoSplit wrote: 
persactly!

 NoEnzLefttoSplit wrote: 
Yeah. That helps.

 BlueHeronDruid wrote: 
morning... I guess it's kind of like the difference between the ping that a good wine glass makes when you flick it with your finger and the dull thwat a cracked wine glass makes.

 geoff_morphini wrote: 
'Splain "rotten rock." I've heard this phrase before, but don't really understand what it means. And yes, my Google finger is borken.

 geoff_morphini wrote:Tectonics huh?  I'll bet your plates full.  (snork...) 

 kurtster wrote: 
Earthquakes in the east tend to feel bigger than in the west.  Although that was "only" a M 3.6 earthquake, the rocks in the east are stronger and less faulted.  It's like having a good baseball bat vs. a broken one.  Here in CA, a M 3.6 would hardly be felt because the energy gets all dissipated in the rotten rock.  That's pretty cool that you felt an injection-induced earthquake...

 geoff_morphini wrote: 
Thanks for the reply.  How wierd it is to see on the reference list that one of the most discussed injection induced quakes is right here in my current back yard at the Perry nuclear plant east of Cleveland.  I remember the quake and was standing on land that was primarily fill at a plaza on the Ohio Turnpike that I managed at the time.  It shook the place pretty good and I remember everyone going WTF was that ?  It sort of brought a smile to my face like an old friend returning and I said it feels like a quake to me and I even got the magnitude right.

I miss the quakes to this day.  But I also know to be careful about what you wish for.

 kurtster wrote: 
Well, I've heard people wonder if oil extraction could induce earthquakes.  There's quite a bit of documentation about injection causing them. For example, one famous occurrence was when the Rocky Mountain Arsenal was injecting chemical wastes into a 12,000+ ft well. The timing and number of earthquakes was directly correlated to the volume and timing of injection events.  I've included some  list of references that, if only by their title, suggest that there is something to the connection between fluid extraction/injection and seismicity.  As for the LA basin, I couldn't tell you.  Most of the oil/gas is extracted from the crests of anticlines (folds) that are bounded by faults.  I would assume that if they extract enough fluid to affect the pore pressure (fluid pressure between grains) near the fault there could be some consequence.

There's less known about extraction-induced References for example.

I was on a panel convened by the city of Berkeley after the 1991 Berkeley Hills fire.  We were asked to come up suggestions about how to study the Hayward fault in the wake of so many houses that had been built on it suddenly disappearing.  I don't think Cordonices Creek was involved in the fire so it is likely standing unless someone did the Bay Area thing and bought it for a tremendous amount of money and then razed it to build something bigger...

 geoff_morphini wrote: 
Ok, here's a question. 

When we moved from NoCal to SoCal in the mid 60's and were able to spend more time with my late Grandmother, she commented on many occasions how the earthquakes in the LA basin had increased in intensity over time (decades).  She believed it was due to the huge amount of oil pumped out and the fact that the basin had sunk some 30 to 50 feet in places due to the oil pumping.  She also believed that the oil in the ground had provided a dampening effect on the earthquakes and that the removal was one of the reasons that quakes originating in the basin had become more violent over time.

Any thoughts on that ?  Always been curious about that one. 

The last house we lived in Berkeley, my Dad built right smack dab on the Hayward fault in the Berkeley hills on Cordonices Creek right above the park of the same name.  It was a redwood house built Japanese style on stilts with cement piers that went 30 to 50 feet straight into the bedrock so it would shake and roll with the quakes and not down the hill side.  When ever I hear of a quake in the East Bay I immediately wonder if the house still stands.

 geoff_morphini wrote: 
I'm going to need time to digest this..

*opens whisky cabinet*  see you guys tomorrow! and cheers geo, truly, this stuff really inspires me.

 geoff_morphini wrote: 
crass overstatement! 

 SantaFeGrace wrote: 
Sorry about that.  I tend to reply to NoEnz in more technical terms since he's got a handle on it.  If there's something you'd like explained I can try to do it in more general terms.

 NoEnzLefttoSplit wrote: 
There are southwest-dipping faults in the area, and they are seismogenic.  It's just that there aren't that many left-lateral faults in the area so the SW-dipping one that would be left-lateral is less likely.  If you look at the SAF in total the bend (see Fig. below) you can see that the bend (between the red arrows) is toward the left.  If you look at the plate motions (which I drew in as green arrows), they are opposed to one another at the bend and result in compression.  The blind part of the fault is just that they were not recognized for many years because they aren't expressed at the surface with a scarp as most faults are.  It becomes complicated.  If you're really interested look under "Blind thrust" and papers by Suppe or Namson and Davis.  They are the pioneers at understanding these types of structures.  The papers are a bit technical though.



Here's a map of the LA basin with focal mechanisms.  Almost all of these, away from the SAF are reverse and show where the blind thrusts are located.  Follow the link (click on the image) for the paper that describes these faults.

 geoff_morphini wrote: 
Sure, you like your rocks shaken... so to speak..
thanks for the explanation. feel free to expound anytime!!. 

 NoEnzLefttoSplit wrote: 
I guess that's why I didn't understand half of it...

 arsenault wrote: 
Thanks, geologist actually, but tectonics is my "thing"

 arsenault wrote: 
he teaches it and yes it's pretty damn impressive.

 geoff_morphini wrote: 
cool stuff..

a southwest dipping plane in that region would be more than unlikely wouldn't it? That would put it perpendicular to all the other faults in the region.  I am still amazed that faults which are essentially caused by large scale tectonic movements can display such tight turns at a local level. I can get my head around fractures and sudden 90° shifts on MORs for instance, but these tight curves are weird. The other one that intrigues me is up around Tonga/Fiji.

Aren't these faults blind precisely because they are buried under (relatively) soft alluvials? i.e. I don't quite get why they would be blind because of the left stepping bend in the SA.