XPost: sci.geo.geology, van.general, sci.geo.earthquakes   
   From: into@oblivion.nothing.com   
      
   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   
   --   
   http://www.skywise711.com - Lasers, Seismology, Astronomy, Skepticism   
   Sed quis custodiet ipsos Custodes?   
      
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