[CEUS-earthquake-hazards] RECURRENCE -Comment on "major" earthquake

[Hempen]

   I apologize that Dr. Stein needed to correct my misplaced emphasis for my point to be clarified.  I should have delayed my comments until properly proofed and then made the request about recurrence.  
   The issue for the recurrence was that the quoted statement ("we don't need to worry about a Major New Madrid earthquake for another 300 years") misrepresents the 500-year return period (Poisson's 9.5% probability of exceedance in 50 years) of a Major event.  The quote suggests that the event is NOT likely to occur until another 300 years have passed.  In fact over the 196 years since the Great New Madrid Series, there was a 32% probability of one or more Major events.  Over the full 500 years until 2312 there is a 63% probability of one or more Major events.  This is much different than the public's perception (and the intent for using the quote) of the binary instance, an Off / On toggle - the improper view that there is almost no chance of such an event until return period has been approached.  
   I grant that 11 times the energy needs to be stored to transform a Strong M6.3 to a Major M7.0.  The 100 - 150 year recurrence that Dr. Stein noted, or 33% probability of exceedance in 50 years, is not an absolute, binary return interval.  The 113 years since the 1895 event had a 60% probability that one or more low M6 or greater events would have occurred.  I would again state that a Strong New Madrid event is overdue.  Yet Palmdale, CA shows that we cannot trust seeming periodicity. 
   So thanks, Dr. Stein, for correcting my misplaced emphasis so that the real argument concerning RECURRENCE that I failed to make could be cited.  
   I ask all to please consider using "x% probability of exceedance for one or more magnitude-specified events in 50 years" (or whatever interval that one would choose).  Like weather forecasts of a "30% chance of rain tomorrow," the public can understand the likelihood of a hazard over a time interval.  
   I appreciate the comments again of Zhenming and restate the following.  With so much to learn about earthquakes, we all should take care to be consistent and not misstate what is understood.  
   [Further, I will heed my own advise to read and correct my own statements before dashing them off.]

Greg 
Greg Hempen, PhD, PE, RG
URS Corporation, greg_hempen at urscorp.com 

  ----- Original Message ----- 
  From: Seth Stein 
  To: Central and Eastern U.S. Earthquake Hazards Listserve 
  Sent: Monday, May 19, 2008 3:42 PM
  Subject: Re: [CEUS-earthquake-hazards] Comment on "major" earthquake


  The term "major" earthquake is typically used in textbooks, etc. for 
  magnitude greater than 7

  http://en.wikipedia.org/wiki/Richter_magnitude_scale

  In the New Madrid zone the last was in 1812. A magnitude 6 occurs on 
  average about every 100-150 years.

  It's interesting to read the accounts of the 1895 (M estimated as 
  6.0-6.6) earthquake, the largest since 1812

  http://earthquake.usgs.gov/regional/states/events/1895_10_31.php

  and the 1968 (M ~ 5.5) earthquake

  http://earthquake.usgs.gov/regional/states/events/1968_11_09.php

  In both, most of the damage occurred in an area close to the epicenter 
  of intensity VII (moderate damage) consisting of "downed chimneys, 
  cracked walls, shattered windows, and broken plaster," and lesser 
  effects further away.

  This is obviously not good, but it's probably not what the public would 
  regard as a "major" disaster. It's worth noting that if everyone 
  involved had had earthquake insurance with a 15% deductible

  http://www.stltoday.com/stltoday/news/columnists.nsf/savvyconsumer/story/FA8B07211D6155BD8625743600105DFE?OpenDocument

  few would have collected.

  Of course one could conceive of locations in the New Madrid zone where 
  if such an earthquake happens to hit damage would be greater

  -- 
  Seth Stein
  William Deering Professor
  Department of Earth and Planetary Sciences
  1850 Campus Drive
  Northwestern University, Evanston, IL 60208
  (847) 491-5265 FAX: (847) 491-8060 E-MAIL: seth at earth.northwestern.edu
  http://www.earth.northwestern.edu/people/seth


  >   
  >    With so much to learn about earthquakes, we all should take care to 
  > be consistent and not overstate what is known.  A quoted seismologist in 
  > discussing the 18 APR 08 Mount Carmel, IL event said "we don't to worry 
  > about a */MAJOR /*[highlighted for emphasis] New Madrid earthquake for 
  > another 300 years."  It is my understanding that we are overdue for a 
  > low M6 event (yet not 100% likelihood in 10 years) in the New Madrid 
  > region.  A major event and a low M6 is too casual of use and certainly 
  > feeds the misinformation to the public, because of a slip of the tongue. 
  >    
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