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<TITLE>Re: [CEUS-earthquake-hazards] no "right" answer</TITLE>
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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 Johnston 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 Johnston, 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 Johnston 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 Memphis <BR>
on the earthquake hazards in the central U.S. 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 Mississippi 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 Memphis 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 Univ. of Memphis.<BR>
<BR>
<BR>
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