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

Wed Feb 20 06:54:15 MST 2008

Art,

Your response to our comment (Wang and others, 2005) was on the ground motion from a single M7.7 New Madrid earthquake with 500 years recurrence interval (an 1811-12 type earthquake).

I agree with you that "as you design buildings to ground motions with lower probability levels, you are protecting a larger fraction of buildings from the range of ground motions expected during the next 1811-12 type earthquake."  This is the basic in decision making.

But my question is what are the probability levels (or confidence levels), 0%, 50%, or 80%, on the national ground motion hazard maps (with 500, 1,000, 2,500 years return periods)? Engineers and policy-makers can make their decisions if the probability levels are known.

Thanks.

Zhenming

________________________________
From: Arthur D Frankel [mailto:afrankel at usgs.gov]
Sent: Tuesday, February 19, 2008 11:01 PM
To: Wang, Zhenming
Cc: ceus-earthquake-hazards at geohazards.usgs.gov
Subject: Re: [CEUS-earthquake-hazards] FW: reply to Joe Tomasello; buildings codes and earthquake hazard

Zhenming,

   These two statements are not contradictory, when taken in the context that I wrote them.

   In the quote from my response to my comment, I was referring to the ground motions observed at any given location over time. At any particular site the ground motions with a 2% chance of being exceeded in 50 years, will occur, on average, once in 2500 years. As I said before, another way to express this is that these ground motions have a 1/2500 chance of being exceed each year.

   The point of my recent email is that each time an 1811-12 type earthquake occurs, there will be some locations that will experience the 2%/50 ground motions or larger, because of the spatial variability of ground motions.  The set of sites that experience these higher ground motions will likely be different for each occurrence of this type of earthquake, because of the variability of ground motions from earthquake to earthquake. This apparent temporal variability of ground motions will occur even for successive earthquakes on the same fault, because of differences in rupture propagation and slip on the fault from earthquake to earthquake.

As you design buildings to ground motions with lower probability levels, you are protecting a larger fraction of buildings from the range of ground motions expected during the next 1811-12 type earthquake.
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<mailto:afrankel at usgs.gov>

-----ceus-earthquake-hazards-bounces at geohazards.usgs.gov wrote: -----
To: "ceus-earthquake-hazards at geohazards.usgs.gov" <ceus-earthquake-hazards at geohazards.usgs.gov>
From: "Wang, Zhenming" <zmwang at email.uky.edu>
Sent by: ceus-earthquake-hazards-bounces at geohazards.usgs.gov
Date: 02/19/2008 07:34AM
Subject: [CEUS-earthquake-hazards] FW: reply to Joe Tomasello; buildings codes and earthquake hazard

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:image001.gif at 01C8739F.F251CAA0]

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 inMemphis 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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