XPost: sci.geo.geology, van.general, sci.geo.earthquakes   
   From: Paul@Houston.com   
      
   Skywise wrote:   
   > Paul in Houston TX  wrote in news:jglbi4$g30$1@dont-   
   > email.me:   
   >   
   >> Quakes happen due to compressive stress along a fault plane   
   >> being relieved suddenly.   
   >> If the stress gets relieved in small movements before it gets   
   >> to the 6.7 state, the 6.7 will never happen.   
   >   
   > Let's try this a different way...   
   >   
   > It's a gross oversimplification, but let's say the stress builds   
   > at a constant rate. Let's say it takes 100 years for the stress   
   > to build up such that it would cause a mag 6.7 quake.   
   >   
   > In order for a number of 5.7's to relieve that stress and prevent   
   > that 6.7 in 100 years, there would have to be 32 of those 5.7's in   
   > that 100 years - a 5.7 every 3 years or so.   
   >   
   > Now, since a 5.7 doesn't happen every three years, the stress is   
   > still building up. Eventually, that 6.7 will still happen.   
   >   
   > Of course, it _is_ much more complex than that. For one, that 5.7   
   > only relieved the stress on a small part of the fault. It did nothing   
   > to relieve it elsewhere. It's even entirely possible that it ADDED   
   > to the stress elsewhere. It may have brought other more stressed   
   > parts of the fault closer to breaking. It _may_ be a foreshock of   
   > things to come. Then again, it may not.   
   >   
   > Let's put in some more real numbers. Again, the Cascadia subduction   
   > zone could generate a magnitude 9 quake. The last one seems to have   
   > been back in 1700. Rounding off that's 300 years ago. Let's say it   
   > takes 500 years to build the stress for a magnitude 9 quake. To   
   > relieve that stress, you'd need 1000 mag 7 quakes, or one million   
   > magnitude 5 quakes. Since we're only 3/5th's into the cycle, there   
   > would have to have been 600 magnitude 7's or 600,000 magnitude 5's   
   > in order to keep up with the stress buildup. Have there been that   
   > many quakes of these sizes since 1700?   
   >   
   > Remember what I first said, each full point of magnitude represents   
   > a 32 fold increase in energy. 2 points is 1000x, 3 points = 32,000,   
   > 4 points = 1 million, 5 points is 32 million.... etc... It's a   
   > logarithmic scale.   
   >   
   > If this one little 5.7 quake relieved any stress on Cascadia, it   
   > only delayed the inevitable 9.0 by a few days, maybe a couple weeks   
   > at best.   
   >   
   > Brian   
      
   That is a great explanation!   
   Thank you for taking the time to write that.   
   Paul   
      
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