Since this list server is read by many, most of whom probably do not
participate in the code process and might only get their information here,
I believe it is necessary to stop the possible spread of the incorrect
statements regarding the USGS maps made in some recent emails on this list
I am referring to an email from Roger Turk on May 25, the second is from me
to Mr. Turk requesting the basis of his May 25 remarks, and a 3rd is from
Mr. Turk on May 31 responding to my request.
On May 25 Mr. Turk stated:
" ... Then, it was determined that Seismic Zones should begin where the
showed those accelerations comparable to Z to begin, forgetting that the
USGS maps reflected accelerations of only 25 percent of what their research
showed would occur. In response to criticisms from SEAOC, USGS brought out
maps that showed the actual accelerations that would occur in California Zone
4 country were actually 3.9g, not .4g. The published reason that USGS gave
for dividing the actual accelerations by 4 was that California Engineers were
experienced in designing for earthquakes and used ductility to accommodate
the difference. (Politics also has an effect on the USGS maps, believe it or
These remarks are incorrect. Consequently, I asked him for his sources in
my May 25 email because I thought he might have misunderstood the source.
On May 31, 2000, Mr. Turk wrote an email with the following information.
"On July 14, 1998, I received a packet of maps with the cover sheet stating:
'NEHRP MAPS 1994 EDITION.'
Maps 6, 8, 10 and 12 show spectral accelerations for parts of California in
excess of 300%g. I recall seeing (an)other map(s) that had accelerations of
close to 4g.
The criticism of the maps by SEAOC was reported in Engineering News-Record.
IIRC, the report was on a BSSC meeting (Kansas City?) and the criticism was
that the maps then published by USGS did not show the actual accelerations
that would be experienced in an earthquake, but 1/4 the actual
accelerations. I have been unable to find my clipping of that article, but I
am sure that it will show up when I least expect it."
These references to the ENR article and the NEHRP maps does not clarify nor
support his remarks about USGS dividing our map values by 4 (or any other
number) nor his comments that USGS divided its values by 4 (or any other
number) in response to SEAOC criticisms. I will expand on this below.
The ENR articles cost me some time at the library to track down the
material. Although I had read the articles when they appeared, I thought it
was important to check them out again before responding. There was a
one-page article entitled "Californians shaky about proposed national
code" and a half-page editorial entitled "Seismic code battle heats up",
both of these were in the February 5, 1996 ENR. The editorial says nothing
about USGS maps. The article says a little (About 5 sentences) about USGS
maps but nothing about criticisms from SEAOC about map values being too
small (or too large). Both pieces contained many factual errors as well as
degrading comments on the BOCA National Building Code and the SBCCI
Standard Building Code. In my view both pieces were way off base in what
they were saying. So much so that they resulted in a March 11 letter to the
editor that said the articles were making "mountains out of molehills".
You can check ENR out for yourselves now that you have the references. You
can make up your own minds about what is written in the article, editorial,
I am not sure why Mr. Turk referred (May 31 email) to the 1994 maps instead
of the 1997 maps but he must have been since only the map numbers in the
1994 map package matches the ones he cited.
Maps 6 and 8 are maps of CA for 10% probability of exceedance in 50 years
of spectral accelerations for 0.3 sec and 1.0 sec respectively. Maps 10
and 12 are maps of CA for 10% probability of exceedance in 250 years of
spectral accelerations for 0.3 sec and 1.0 sec respectively. (Note that
10% probability of exceedance in 250 years is very close to 2% probability
of exceedance in 50 years that is now used in the latest USGS maps.) Of
course the numbers on the 10% in 50 year maps are smaller than the numbers
on the 10% in 250 year maps. That is to be expected. On the short period
10% in 250 year maps the spectral values reach 3.0 g, but what is the
point? These are spectral accelerations so numbers of this magnitude are
not unusual. I can not speculate on what numbers Mr. Turk is comparing to
arrive at his factor of 4.
It is absolutely incorrect to say that the USGS maps were divided by 4 or
any other number to account for ductility. The code equations account for
ductility (with R factors in its current form) and these factors are not
always 4, nothing new here. I repeat - No USGS maps of any ground motion
parameter on a map we have published at any probability level have ever
been divided by a factor to account for ductility. Accordingly the
"published reason that USGS gave for dividing the actual accelerations by
4" that Mr. Turk referred to does not exist because it was never done.
Needless to say, Mr. Turk's March closing sentence about politics and USGS
maps also has no basis in fact.
If you want to look at the numbers on the probabilistic or design maps for
yourselves, they are on our Web site - http://geohazards.cr.usgs.gov/eq
Finally I would make the comment that this list server has a great
opportunity to serve the engineering community; however, in my view this
opportunity also carries a responsibility to pass on information that is
correct (with references if it is in an unfamiliar area to most so it can
be followed up by those that want to do so) or if it is a best
recollection, that is fine, just say so. Memories certainly faid with
time. Or say so when something is a personal viewpoint. This seems to me
to be particularly important when the information is in an area that many
readers do not usually work in, such as the code process. Many of the
users of this list server do provide such backup material and that
information has been valuable, but it has been my observation that such
backup material has not always been provided on some important issues.
USGS, National Seismic Hazard Mapping Project.
Edgar V. Leyendecker Telephone: 303-273-8565
U.S. Geological Survey Fax: 303-273-8600
Address for mail: Physical address:
Denver Federal Center 1711 Illinois Avenue, Room 425
MS 966, Box 25046 Golden, CO 80401
Denver, CO 80225-0046