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<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>It is clear from these discussions that two
fundamentally different things, earthquake and probabilistic ground motion
(derived from PSHA), are being compared. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Here is a simple question for every one:
If an M7.7 earthquake occurs every 500 years in the New Madrid seismic zone,
how often will the ground motion generated by that earthquake be observed in <st1:City
w:st="on">Memphis</st1:City>, <st1:City w:st="on">St. Louis</st1:City>, or <st1:City
w:st="on"><st1:place w:st="on">Paducah</st1:place></st1:City>?<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Thanks.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'>Zhenming <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 color=navy face=Arial><span style='font-size:
10.0pt;font-family:Arial;color:navy'><o:p> </o:p></span></font></p>
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<p class=MsoNormal><b><font size=2 face=Tahoma><span style='font-size:10.0pt;
font-family:Tahoma;font-weight:bold'>From:</span></font></b><font size=2
face=Tahoma><span style='font-size:10.0pt;font-family:Tahoma'>
ceus-earthquake-hazards-bounces@geohazards.usgs.gov
[mailto:ceus-earthquake-hazards-bounces@geohazards.usgs.gov] <b><span
style='font-weight:bold'>On Behalf Of </span></b>Arthur D Frankel<br>
<b><span style='font-weight:bold'>Sent:</span></b> Wednesday, February 06, 2008
1:55 PM<br>
<b><span style='font-weight:bold'>To:</span></b>
ceus-earthquake-hazards@geohazards.usgs.gov<br>
<b><span style='font-weight:bold'>Subject:</span></b> Re:
[CEUS-earthquake-hazards] alternative hazard maps</span></font><o:p></o:p></p>
</div>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><br>
</span></font><font size=2 face=sans-serif><span style='font-size:10.0pt;
font-family:sans-serif'>Seth,</span></font> <br>
<br>
<font size=2 face=sans-serif><span style='font-size:10.0pt;font-family: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></font>
<br>
<br>
<font size=2 face=sans-serif><span style='font-size:10.0pt;font-family: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></font> <br>
<br>
<font size=2 face=sans-serif><span style='font-size:10.0pt;font-family: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></font> <br>
<br>
<font size=2 face=sans-serif><span style='font-size:10.0pt;font-family: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 <st1:country-region
w:st="on">U.S.</st1:country-region> and southeast <st1:country-region w:st="on">Canada</st1:country-region>,
compared to the USGS map, when you change the magnitude and add time dependence
for the New Madrid and <st1:place w:st="on"><st1:City w:st="on">Charleston</st1:City></st1:place>
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 <st1:City
w:st="on"><st1:place w:st="on">Charleston</st1:place></st1:City> 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 <st1:City
w:st="on">Charleston</st1:City> would affect the hazard calculated for the
northeast <st1:country-region w:st="on"><st1:place w:st="on">U.S.</st1:place></st1:country-region>
</span></font><br>
<br>
<font size=2 face=sans-serif><span style='font-size:10.0pt;font-family: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 <st1:State
w:st="on"><st1:place w:st="on">California</st1:place></st1:State> 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></font> <br>
<br>
<font size=2 face=sans-serif><span style='font-size:10.0pt;font-family: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></font> <br>
<br>
<font size=2 face=sans-serif><span style='font-size:10.0pt;font-family: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></font><br>
<br>
<br>
<font size=2 face=sans-serif><span style='font-size:10.0pt;font-family:sans-serif'>Art
Frankel<br>
<st1:country-region w:st="on"><st1:place w:st="on">U.S.</st1:place></st1:country-region>
Geological Survey<br>
MS 966, Box 25046<br>
DFC<br>
<st1:place w:st="on"><st1:City w:st="on">Denver</st1:City>, <st1:State w:st="on">CO</st1:State>
<st1:PostalCode w:st="on">80225</st1:PostalCode></st1:place><br>
phone: 303-273-8556<br>
fax: 303-273-8600<br>
email: afrankel@usgs.gov</span></font><o:p></o:p></p>
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