[CEUS-earthquake-hazards] reccurrence - Comment on China quake and hazards map

[Hempen]

   Klaus & Zhenming make a very important point of care in using the single value of return period of one or more events.  Not only is this misleading to the public, the single value of return period of one or more events is misleading to owners, public officials, and those with casual use of statistics.  
   It is much better in my opinion to use a x% chance in 50 years of one or more events at a given magnitude.  This is a fully explained concept for multiyears without giving the misimpression that there will NOT be another event over the single value of return period of one or more events.  
   While we may slip into more casual use, it only needs to be misused and spread by a single newspaper or television reporter to undo a great deal of effort.  The public still misunderstands that the odds of a tail in the next perfect coin toss after ten tosses of a head remains 50%.  
   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.  
   It is not my intent to castigate anyone.  I just suggest that we state very carefully our comments particularly for public consumption.  
   Thanks, Zhenming and Klaus, for noting the issue of recurrence.  

Regards,
Greg 
Greg Hempen, PhD, PE, RG

  ----- Original Message ----- 
  From: Wang, Zhenming 
  To: jacob at ldeo.columbia.edu ; Central and EasternU.S. Earthquake Hazards Listserve 
  Cc: olboyd at usgs.gov 
  Sent: Sunday, May 18, 2008 6:39 AM
  Subject: Re: [CEUS-earthquake-hazards] Comment on China quake and hazards map


  Klaus,

  I agree with you that "Hence this earthquake is VERY pertinent to the discussion of how to map seismic
  hazards in regions with long recurrence periods for similar-sized earthquakes, New Madrid included."

  Attached is the Chinese national seismic design map (1956 intensity map). This historical intensity map could save some life if it was used.

  The problem may be the return period defined in PSHA. 500-year return period derived from a PSHA study is just a numerical number and is not equal to any recurrent interval of earthquake. Comparing return period of PSHA with recurrent interval of earthquake is misleading.

  Thanks.

  Zhenming


  ________________________________________
  From: jacob at ldeo.columbia.edu [jacob at ldeo.columbia.edu]
  Sent: Friday, May 16, 2008 10:43 PM
  To: Central and Eastern U.S. Earthquake Hazards Listserve; Wang, Zhenming
  Cc: ceus-earthquake-hazards at geohazards.usgs.gov; olboyd at usgs.gov
  Subject: Re: [CEUS-earthquake-hazards] Comment on China quake and hazards map

  All:

  Thanks Zhenming for the map.

  >From what I can decipher from the map, the region of the M7.9 earthquake is
  shown on the 10% in 50 years map as having PGAs between 10 and 20%g. Although I
  have not heard any strong motion reports, I would guess that much of the
  region, especially on the hanging wall side (the uplands, not in the basin)
  may have seen PGA's of 0.5g and possibly larger.

  The discrepancies between map values and likely real PGAs are most likely due to
  the following:

  >From what little I know about the region's geology and seismic history, there is
  no earthquakes as large as this one in the historic record. Geological mapping
  of faults has (see cooperative work between Burchfield's group at MIT
  and many Chinese geologists) seem to NOT have found recently active SURFACE
  faulting in the area, and what they found were seemingly older faults largely
  with strike slip components (while this quake is largely a thrust with minor
  strike slip).

  This mapping may have missed that there could be a blind thrust with no surface
  fault. But the topographic front looks formidable and needs relatively recent
  thrusting/reverse faulting, blind or not blind.

  The point is: if there was no large historic quake in the historic record, no
  recognized fault with measurable slip rate, and low geodetic strain rates, then
  the topography should have been a warning, albeit allowing for VERY LONG
  RECURRENCE PERIODS OF SEVERAL 1000 YEARS for events on this fault or thrust
  belt. If the recurrence period is this long, then it is hardly surprising that
  on the hazards map for 10% in 50 years (average recurrence period 475 years)
  this does zone does not show up very prominently.

  This is exactly the reason why some time ago the US NEHRP hazard maps (i.e.
  USGS maps) started to portray 2% in 50 years (2475 years recurrence period), to
  catch regions like this with reasonably "safe" ( speak high) PGA values, or
  sufficiently high spectral acceleration values for building code applications
  (like in the CEUS).

  Hence this earthquake is VERY pertinent to the discussion of how to map seismic
  hazards in regions with long recurrence periods for similar-sized earthquakes,
  New Madrid included.

  Of course the tectonics is entirely different, but there are lessons to be
  learned, and pertinent to the issues we all discussed with such passion in this
  forum (but on different sides of the fence) a few month ago.

  Of course there is the other issue about seismic building design and quality
  control) or lack of both, but if there would have been full quality control,
  the 10% in 50 year map PGA values don't provide sufficient protection.

  And that is the lesson of this Eq. for the CEUS, and perhaps China will change
  its code after this quake to longer recurrence periods, closer to what we have
  for the US right now (2% in 50 y, or at least 2/3 of these hazard levels !!!!).

  Best
  Klaus Jacob
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