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<title>Re: [CEUS-earthquake-hazards] no "right" answer</title>
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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'>Jose,<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'>First, I want to make clear that the Kentucky
Geological Survey has strongly opposed the ways that the input parameters, particularly
the magnitude, recurrence interval, and fault location for the New Madrid
seismic zone, were determined in the national seismic hazard mapping. The
reason to use the input parameters of the national hazard mapping for
discussions on this listserver is for easy comparisons. <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'>You have brought up many outstanding
issues in terms of earthquake science in the New Madrid seismic zone. However, these
issues may not be relevant to the national seismic hazard maps because of the
way that the scientific parameters (including uncertainties) are combined
through the methodology - probabilistic seismic hazard analysis (PSHA). The end
results from the national seismic hazard mapping are hazard curves that provide
a range of ground motion (0 to 10g or larger) vs. annual probability of exceedance
(or return period). It is difficult to compare the hazard curves or the points
from the curves. <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'>The mean magnitude and recurrence interval
of M8.0 and 1,000 years were used in the 1996 national hazard maps, while M7.7
and 500 years were used in the 2002 national hazard maps. Other different parameters
including ground motion attenuation relationships and logic trees were also used
the 1996 and 2002 maps. The PGA value is about twice larger on the 2002 map than
that on the 1996 map both with 500-year return period. The PGA value is about the
same on the 2002 and the 1996 maps both with 2,500-year return period. If
comparing the 2,500-year maps, they are quite similar although the input
parameters are quite different. <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'> Jose M Pujol
(jpujol) [mailto:jpujol@memphis.edu] <br>
<b><span style='font-weight:bold'>Sent:</span></b> Tuesday, February 19, 2008
2:19 PM<br>
<b><span style='font-weight:bold'>To:</span></b> <st1:PersonName w:st="on">Wang,
Zhenming</st1:PersonName><br>
<b><span style='font-weight:bold'>Cc:</span></b> jacob@ldeo.columbia.edu; <st1:PersonName
w:st="on">ceus-earthquake-hazards@geohazards.usgs.gov</st1:PersonName><br>
<b><span style='font-weight:bold'>Subject:</span></b> Re:
[CEUS-earthquake-hazards] no "right" answer</span></font><o:p></o:p></p>
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<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 style='margin-bottom:12.0pt'><font size=3
face="Times New Roman"><span style='font-size:12.0pt'>I appreciate the
opportunity offered by this listserve to express my concerns<br>
regarding the NMSZ probabilistic hazard analysis, summarized below.<br>
Before proceeding, however, I want to make clear that I am not siding <br>
with anybody involved in the ongoing debate, most of whom I don't know
personally. <br>
<br>
<br>
1) I have some problems with the approach used to select the magnitude of
7.7<br>
as the characteristic magnitude of the 1811-1812 New Madrid earthquakes. The <br>
logic tree described in the 2002 USGS Open-File report 02-420 is based on the <br>
following magnitudes and weights (in parentheses): 7.3 (0.15), 7.5 (0.2),
<br>
7.7 (0.5), 8.0 (0.15). This scheme produces a mean hazard essentially <br>
equivalent to that obtained by giving full weight to the magnitude 7.7. <br>
This scheme was developed using input from a workshop and results from Bakun <br>
and Hopper that determined a magnitude range of 7.0-7.7, and includes the <br>
magnitude 8 from <st1:City w:st="on"><st1:place w:st="on">Johnston</st1:place></st1:City>
results (Geophys. J., 1996) and the lower <br>
magnitudes determined by Hough et al. (JGR, 2000). I don't know why the <br>
magnitude 7.7 was given the highest weight, but what I think is missing here <br>
is a serious discussion of the arguments made by Hough et al. Their results <br>
are essentially incompatible with those of <st1:City w:st="on"><st1:place
w:st="on">Johnston</st1:place></st1:City>, and it seems to me that <br>
it is not scientifically sound to treat their respective magnitudes as if they <br>
were samples from a random variable with weights that are assigned
subjectively. <br>
<br>
At the risk of oversimplifying things, the case made by Hough et al. is that <br>
the strongest effects of the 1811-1812 earthquakes were felt in river valleys, <br>
while hard-rock sites were much less affected. This clearly point to strong <br>
site effects. Everybody knows that unconsolidated sediments can increase the <br>
amplitudes of ground motion several fold with respect to a rock site. In
addition, <br>
the duration of shaking also increases, which is significant for the
following <br>
two reasons. First, damage to buildings and other structures increases when <br>
the duration of an earthquake increases. For example, a building may withstand <br>
one of two cycles of strong shaking, but not several cycles of it. Second, <br>
the possibility of liquefaction increases with duration because an increase <br>
in the number of stress cycles lowers the intensity required for failure. <br>
Damage has been used to assign intensities and liquefaction arguments have <br>
been used to justify the larger magnitudes assigned to the 1811-1812 events.<br>
<br>
It seems obvious to me that site effects must be taken into account, as
proposed <br>
by Hough et al. This is why I don't think that it is scientifically sound to
treat<br>
Hough et al. and <st1:City w:st="on"><st1:place w:st="on">Johnston</st1:place></st1:City>
results as more or less equally probable. In addition, <br>
the Bakun and Hopper (BSSA, 2004) larger magnitude estimates were obtained
using <br>
their own intensity estimates; when the Hough et al. estimates are used the <br>
magnitudes end up being somewhat smaller. In 2005 there was a meeting in <st1:City
w:st="on">Memphis</st1:City> <br>
on the earthquake hazards in the central <st1:country-region w:st="on"><st1:place
w:st="on">U.S.</st1:place></st1:country-region> organized by the Applied
Technology <br>
Council and the USGS. One of the speakers was B. Schweig, who noted that the <br>
1811-1812 earthquakes had "probably at least magnitude 7.5, and they shook
like <br>
magnitude 8.0". This statement clearly summarizes the uncertainty
affecting the <br>
magnitudes estimates. <br>
<br>
Another point I want to make is that several other papers published after <br>
the Hough et al. paper are consistent with their results (see Hough et al., <br>
SRL, 2005, v. 76, 373-386). As a disclaimer, this includes a paper written by <br>
Mueller and me (BSSA, 2001, v. 91, 1563-1573), where we also argue for somewhat
<br>
lower magnitudes. Yet, the USGS sticks to its 7.7 magnitude.<br>
<br>
In any case, what I think is needed is to investigate the magnitude problem <br>
using a more scientific approach. I proposed it in a proposal submitted to the <br>
USGS, and although the panel did not recommend funding, there was no objection <br>
to the basic principles, which are rather simple. Because intensity
measurements <br>
are actually a proxy for the ground motion during an earthquake, the
effect of <br>
the unconsolidated sediments of the <st1:State w:st="on"><st1:place w:st="on">Mississippi</st1:place></st1:State>
embayment can be inferred from <br>
analysis of synthetic data. The idea is to generate synthetic seismograms for <br>
embayment and hard-rock models, and to estimate magnitudes applying standard
methods<br>
to the synthetic data. A comparison of results for the two types of sites will <br>
give us a good idea of the of the effect of the sediments on magnitude
estimation.<br>
Preliminary work I did seems to indicate that it may introduce a difference<br>
in magnitude of about 0.5. Of course, more work is needed, but since I am
currently <br>
working on other projects, I hope somebody else may be interested in pursuing
my <br>
ideas.<br>
<br>
<br>
2) I have not followed carefully the arguments on the probability of occurrence
<br>
of a magnitude 7.7 earthquake in the near future, but what I found striking is <br>
the statement that there is a 7-10% probability that a magnitude 7.7 earthquake<br>
can occur within any 50 year period. This was big news in the <st1:City w:st="on"><st1:place
w:st="on">Memphis</st1:place></st1:City> paper <br>
in 2005 (Commercial Appeal, Dec. 11). If I understand this correctly, it means <br>
that there is a significant probability that the strain accumulation required
for <br>
such an earthquake will be reached by 2055 at the latest. This is about 250
years <br>
from the 1811-1812 earthquakes. If the recurrence rate is 500 years, my <br>
interpretation is that a repeat of one of them requires a significant increase <br>
in the rate of strain accumulation. This in turn will require a significant<br>
acceleration in the physical process(es) responsible for the strain in the
NMSZ. <br>
So I wonder whether the 7-10% prediction is based on purely probabilistic <br>
considerations, or whether it also uses other information, such as GPS. I would
<br>
appreciate it very much if someone could clarify this for me.<br>
<br>
Jose Pujol<br>
Professor<br>
Dept. of Earth Sciences<br>
The <st1:place w:st="on"><st1:PlaceType w:st="on">Univ.</st1:PlaceType> of <st1:PlaceName
w:st="on">Memphis</st1:PlaceName></st1:place>.<br>
<br>
<o:p></o:p></span></font></p>
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