[CEUS-earthquake-hazards] alternative hazard maps

[Joe Tomasello]

Art, et al:

 

Regarding item 1 below: The statement indicates that USGS has been
contracted only to make maps, not policy.  Why then does it appear before
state commissions advocating only using 2%PE in 50 years? Isn't the 10% PE
just as valid? You, yourself, have appeared before at least one commission
in Tennessee, not to mention CERI. At that time, representing USGS, you
advocated the use of the 2% PE map over any other.  This is why most of us
in the private sector understand USGS's policy to be one for strong mandated
seismic mitigation. That is USGS policy, isn't it? 

 

 

Joseph Tomasello, PE

5880 Ridge Bend Rd.

Memphis, TN 38120

 

Phone:

(901) 761-2016 office

(901) 821-4968 direct

(901) 412-8217 mobile

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: Wednesday, February 06, 2008 12:55 PM
To: ceus-earthquake-hazards at geohazards.usgs.gov
Subject: Re: [CEUS-earthquake-hazards] alternative hazard maps

 


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

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