[CEUS-earthquake-hazards] FW: reply to Joe Tomasello; buildings codes and earthquake hazard

[Wang, Zhenming]

Here is another inconsistent statement on the national seismic hazard maps.

"You have the mistaken notion that 2%/50 values are only observed once in 2500 years. This is flat out wrong."

In a response to our comment (Wang and others, 2005) that was published on Seismological Research Letter (Frankel, 2005), the interpretation was "the ground motion with 2% PE in 50 years is exceeded once, on average over 2,500 years, so that it has a 1/2500 annual probability of being exceeded."

 [cid:image002.gif at 01C872DC.7612BC10]

As demonstrated earlier, for a single M7.7 earthquake with 500 year recurrence interval in the New Madrid seismic zone, ground motion with 2,500-year return period (2% PE in 50 years) means there is about 20 percent probability that ground motion will be exceeded if the M7.7 earthquake occurs. In other words, if the ground motion with 2,500-year return period is selected for engineering design, we has a confidence level of 80% (not being exceeded) if the M7.7 earthquake occurs.

Thanks.

Zhenming

________________________________
From: ceus-earthquake-hazards-bounces at geohazards.usgs.gov [mailto:ceus-earthquake-hazards-bounces at geohazards.usgs.gov] On Behalf Of Arthur D Frankel
Sent: Friday, February 15, 2008 1:50 PM
To: Joe Tomasello
Cc: ceus-earthquake-hazards at geohazards.usgs.gov; mpetersen at usgs.gov
Subject: Re: [CEUS-earthquake-hazards] reply to Joe Tomasello; buildings codes and earthquake hazard


Joe,

  I feel I need to reply to you on the bulletin board, since you seem to want to keep this discussion going and you directly ask me questions. My apologies to Paul Segal and others who are fed up with this.

  I guess you missed one of the key points of my previous email.   I said that the IBC values (2/3 times the ground motions with 2% probability of exceedance in 50 years [2%/50]) were probably experienced in Memphis during the 1811-12 earthquakes, based on intensity data from those earthquakes.

   You have this mistaken notion that IBC values (2/3 times the 2%/50 motions)  are only observed once in a millenium.  You have the mistaken notion that 2%/50 values are only observed once in 2500 years. This is flat out wrong.    Some locations will experience 2/3 times the 2%/50 ground motions during the next 1811-12 type earthquake.   Some locations will experience the 2%/50 ground motions during the next 1811-12 type earthquake.    This is a simple consequence of the observed variability of earthquake ground motions.

    As far as the magnitude issue you bring up, the USGS uses the range of magnitudes that various seismologists have determined  for the 1811-12 earthquakes from the intensity data.  The central values of these moment magnitude determinations range from 7.4-7.5  (Hough et al., 2000 in JGR) to 7.8 (Bakun and Hopper, 2004 in BSSA)  to 8.0-8.1 (Johnston, 1996 in Geophysical Journal).  In the national maps we use a logic tree to express this range, with a value of 7.7 given the highest weight.   I used M7.7 in the scenario in my previous email, because it is in the center of the range of magnitudes determined for the 1811-12 earthquakes.  I also gave results for a M7.4 earthquake.

    In the example in my previous email,  I placed the scenario earthquake where the current seismicity trend for the New Madrid seismic zone is located. The closest distance to downtown Memphis is about 60 km.

-Art




Art Frankel
U.S. Geological Survey
MS 966, Box 25046
DFC
Denver, CO 80225
phone: 303-273-8556
fax: 303-273-8600
email: afrankel at usgs.gov

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