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<p class=MsoNormal>Art, et al:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>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? <o:p></o:p></p>
<p class=MsoNormal><span style='font-size:11.0pt;color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><b><span style='font-family:"Arial","sans-serif";color:#1F497D'>Joseph
Tomasello, PE</span></b><span style='color:#1F497D'><o:p></o:p></span></p>
<p class=MsoNormal><b><span style='font-family:"Arial","sans-serif";color:#1F497D'>5880
Ridge Bend Rd.</span></b><span style='color:#1F497D'><o:p></o:p></span></p>
<p class=MsoNormal><b><span style='font-family:"Arial","sans-serif";color:#1F497D'>Memphis,
TN 38120</span></b><span style='color:#1F497D'><o:p></o:p></span></p>
<p class=MsoNormal><span style='color:#1F497D'> <o:p></o:p></span></p>
<p class=MsoNormal><b><span style='font-family:"Arial","sans-serif";color:#1F497D'>Phone:</span></b><span
style='color:#1F497D'><o:p></o:p></span></p>
<p class=MsoNormal><b><span style='font-family:"Arial","sans-serif";color:#1F497D'>(901)
761-2016 office</span></b><span style='color:#1F497D'><o:p></o:p></span></p>
<p class=MsoNormal><b><span style='font-family:"Arial","sans-serif";color:#1F497D'>(901)
821-4968 direct</span></b><span style='color:#1F497D'><o:p></o:p></span></p>
<p class=MsoNormal><b><span style='font-family:"Arial","sans-serif";color:#1F497D'>(901)
412-8217 mobile</span></b><span style='font-size:11.0pt;color:#1F497D'><o:p></o:p></span></p>
<div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'>
<p class=MsoNormal style='margin-left:.5in'><b><span style='font-size:10.0pt;
font-family:"Tahoma","sans-serif"'>From:</span></b><span style='font-size:10.0pt;
font-family:"Tahoma","sans-serif"'>
ceus-earthquake-hazards-bounces@geohazards.usgs.gov
[mailto:ceus-earthquake-hazards-bounces@geohazards.usgs.gov] <b>On Behalf Of </b>Arthur
D Frankel<br>
<b>Sent:</b> Wednesday, February 06, 2008 12:55 PM<br>
<b>To:</b> ceus-earthquake-hazards@geohazards.usgs.gov<br>
<b>Subject:</b> Re: [CEUS-earthquake-hazards] alternative hazard maps<o:p></o:p></span></p>
</div>
<p class=MsoNormal style='margin-left:.5in'><o:p> </o:p></p>
<p class=MsoNormal style='margin-left:.5in'><br>
<span style='font-size:10.0pt;font-family:"Arial","sans-serif"'>Seth,</span> <br>
<br>
<span style='font-size:10.0pt;font-family:"Arial","sans-serif"'> 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.</span> <br>
<br>
<span style='font-size:10.0pt;font-family:"Arial","sans-serif"'>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.</span> <br>
<br>
<span style='font-size:10.0pt;font-family:"Arial","sans-serif"'>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.</span> <br>
<br>
<span style='font-size:10.0pt;font-family:"Arial","sans-serif"'>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. </span><br>
<br>
<span style='font-size:10.0pt;font-family:"Arial","sans-serif"'>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.</span> <br>
<br>
<span style='font-size:10.0pt;font-family:"Arial","sans-serif"'>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).</span> <br>
<br>
<span style='font-size:10.0pt;font-family:"Arial","sans-serif"'>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. </span><br>
<br>
<br>
<span style='font-size:10.0pt;font-family:"Arial","sans-serif"'>Art Frankel<br>
U.S. Geological Survey<br>
MS 966, Box 25046<br>
DFC<br>
Denver, CO 80225<br>
phone: 303-273-8556<br>
fax: 303-273-8600<br>
email: afrankel@usgs.gov</span><o:p></o:p></p>
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