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

Mon May 19 18:23:56 GMT 2008

Zhenming,

First, thank you very much for the maps. (Also thanks to Rogers for the fault-quake maps).
I see the 1956 intensity map assigns a MMI ~ IX in the zone of 2008 earthquake.
Indeed, using this map  (+ good structural designs) could save some life.

Regarding the PGA map, I think the problem is not the mean return period for designing, 
but the seismogenic source model used for developing the map.

In the slides from Prof. Steve Gow one can see the 1713 earthquake on the fault.
I attach a small map with this earthquake and others (1713, 1748, 1792 and 1793 earthquakes);
data taken from Ganse & Nelson's catalog (BSSA, vol. 72, pp. 873-877, 1992).
According this catalog, the 1713 earthquake reached MMI=VIII and the 1748 earthquake MMI=VII.
Now, according to Wald et al., (EQS, Vol. 15, pp. 537-555, 1999) for a VIII earthq. one can 
expect a PGA= 0.34 - 0.65 g, and for a VII earthquake PGA= 0.18 - 0.34 g.  Both >> 0.1 g.

The 1713 earthq. happened 295 years ago. Of course, a difficulty is the unknown previous earthquake.
But, consideration of this earthquake must lead to expect a high PGA for a 475 years mean return period.
It is difficult to understand why a PSHA leads to ~0.1 g for this period.
For a site practically on the fault, which seismogenic parameters are supposed for obtaining a such PGA?
Because of these observations, I think the seismogenic source model used was inexplicably wrong.

Regards, 
_______________________________________________________
Julio J. Hernández 
Consultant on Structural Engineering and Earthquake Engineering 
Caracas, Venezuela 
e-mail:  julher at cantv.net 
cellphone:  +58-414-1293989

  ----- 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 7:39 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
  ==================

  Quoting "Wang, Zhenming" <zmwang at email.uky.edu>:

  > I would like to share with you all the Chinese National seismic design map
  > (PGA with 10% PE in 50 years). This may explain one of the reasons that so
  > many schools and hospitals collapsed in the Wenchuan earthquake.
  >
  > Thanks.
  >
  > ___________________________________
  > Zhenming Wang, PhD, PE
  > Head, Geologic Hazards Section
  > Kentucky Geological Survey
  > 228 Mining and Mineral Resources Building
  > University of Kentucky
  > Lexington, Kentucky 40506
  > Phone:(859)257-5500x142
  > Email: zmwang at uky.edu<mailto:zmwang at uky.edu>
  > Website:
  > www.uky.edu/KGS/geologichazards<http://www.uky.edu/KGS/geologichazards>
  > ____________________________________
  >
  >

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