[CEUS-earthquake-hazards] alternative hazard maps

[Arthur D Frankel]

Seth,

  I have some quick comments on your paper with James Hebden that I think 
would also be of interest to members of the CEUS hazards bulletin board.

1) On page 3 of your paper,  you mistakenly claim that the national 
seismic hazard maps (i.e., Frankel et al. 1996, 2002) define "the hazard" 
at 2% probability of exceedance in 50 years.  You state "Frankel et al. 
(1996; 2002) define the hazard as the maximum shaking predicted at a 
geographic point with 2% probability of exceedance in 50 years, or about 
once in 2,500 years."   Actually, the USGS makes national seismic hazard 
maps at a variety of probability levels, based on scientific information 
such as earthquake recurrence rates and ground-motion attenuation 
relations. In fact, we release seismic hazard curves for a grid of sites 
across the nation, so that users can calculate the ground motions at any 
probability level they choose.   I assume you are referring to the 2/3 
times the  2% probability of exceedance in 50 year level that is used in 
seismic DESIGN maps in the NEHRP Recommended Provisions written by the 
Building Seismic Safety Council, published by FEMA, and adopted in the 
International Building Code (IBC) and the ASCE standards.   This 
probability level for design was not decided by the U.S. Geological 
Survey.     This probability level and design procedure were decided by a 
group of engineers under the Building Seismic Safety Council (funded by 
FEMA) and voted on and approved by a wide set of engineers and engineering 
groups.   It is based on their engineering judgement of acceptable risk. 
It's also important to note that in some areas of the country the design 
maps are based on a deterministic calculation of the median ground motions 
for a characteristic earthquake on a specific fault.  In the 2006 IBC, for 
example, the design values around the New Madrid area are based on the 
median ground motions calculated for a M7.7 earthquake, averaging five 
different attenuation relations.

2) In the same sentence of your paper you say "the maximum shaking... with 
2% probability of exceedance in 50 years..."   This is not correct.  It is 
not the maximum shaking.  Probabilistic ground motions are the ground 
motions with a specified probability of being exceeded.   They are not the 
maximum shaking.   It should also be reiterated that the national seismic 
hazard maps are based on the average hazard curves from a variety of input 
models and attenuation relations; they are not worst-case maps.

3) There seems to be something wrong with some of your calculations. In 
your Figure 7, you show significant changes to the seismic hazard in the 
northeast U.S. and southeast Canada, compared to the USGS map, when you 
change the magnitude and add time dependence for the New Madrid and 
Charleston sources.   The changes in your hazard maps extend past 1000 km 
from these sources. It is very unlikely that the changes you made in New 
Madrid and Charleston would significantly affect the hazard at these 
distances.   As you probably know, we use a 1000 km maximum distance when 
calculating the hazard in the CEUS for the national maps, so there is no 
way changes in New Madrid and Charleston would affect the hazard 
calculated for the northeast U.S. 

4) You use a Gaussian distribution of recurrence times, rather than the 
log-normal distribution or Brownian Passage Time model that are typically 
used in modern earthquake probability studies, such as the Working Group 
on California Earthquake Probabilities (WGCEP, 1995 and 2002). The 
coefficient of variation (COV; standard deviation divided by the mean 
recurrence time) is very important in calculating time-dependent 
probabilities and is a source of uncertainty. Values centered at 0.5 are 
often assigned the highest weight in California probability studies (e.g., 
WGCEP, 1995. 2002), reflecting the substantial variation in recurrence 
times that are observed in many areas that have long enough paleo-event 
chronologies.

5) Using a time-dependent model with a log-normal distribution of 
recurrence times with a COV of 0.5, the USGS calculated a 7% probability 
of a 1811-12 type New Madrid earthquake in the next 50 years, as opposed 
to the 10% probability found from the time-independent model.  This 
probability range (7-10%) was stated in the USGS fact sheet on New Madrid 
(FS-131-02).

6) Of course, key questions are whether a time-dependent model is 
appropriate for an intraplate area and what distribution of recurrence 
times and COV to use in a probability calculation for these areas.  As 
many studies have shown, when a large earthquake occurs on one fault it 
can increase the stress on nearby faults and increase the probability of 
having an earthquake on these faults.  So a time dependent model where the 
hazard in a region is zero right after a large earthquake is very naive 
(it also ignores aftershocks). We know the New Madrid source zone is 
actually a fault system rather than a single fault and we might expect a 
complicated pattern of loading and unloading not described by the simple 
time dependent model used in your paper.  In addition, intraplate fault 
systems are not loaded in the same way as faults along plate boundaries, 
which are being continually loaded by the displacements of tectonic 
plates. 


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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://geohazards.usgs.gov/pipermail/ceus-earthquake-hazards/attachments/20080206/1ef0fa3e/attachment.html