[CEUS-earthquake-hazards] alternative hazard maps

[Joe Tomasello]

We reviewed the commentary some years ago and found their arguments weak.
The primary objective, according the Provision's Commentary, is to provide
"uniform margin against collapse" across the country. The way this is done
is to increase the design seismicity to a level equal to the maximum
probable.  In my opinion, the change from using 10% PE in 50 years to 2% PE
in 50 years is overly conservative. The Provisions justified this by
normalizing New Madrid Accelerations/Frequency curves with California's.
(The SDPG could have easily normalized the Acceleration/Frequency cures
around the 500 year recurrence in the New Madrid Seismic Zone - the choice
was arbitrary.) In addition, wanting a "seismic safety factor" of 1.5 the
SDPG selected the 2%PE maps. However, the commentary goes on to explain that
this would have increased seismic forces in California - which was found
objectionable by California engineers. So, the 2/3 reduction factor was
interjected into the methodology. (refer to FEMA 303a, pages 288-292) The
intent of the reduction factor was to bring the seismicity down to the 10%
PE levels in California. The commentary didn't discuss the fact that this is
3.3 times as much as the 10%PE levels in the New Madrid Seismic Zone.  I'll
go a step further, the only place where the reduction factor brings design
seismicity to the 10% PE levels is on the west coast.  This becomes
economically burdensome in a region that may not see these seismic forces in
one or two millennium. 

If we continue the same logic (more is better) why don't we design all
buildings everywhere in the US for an M9 within 50 miles of the building
site ("it could happen tomorrow"). It would make every jurisdiction uniform.
The simple answer to this logic is that it's not economically feasible to
do. The same holds true with the NEHRP Provisions.

Joseph Tomasello, PE
5880 Ridge Bend Rd.
Memphis, TN 38120
 
Phone:
(901) 761-2016 office
(901) 821-4968 direct
(901) 412-8217 mobile

-----Original Message-----
From: jacob at ldeo.columbia.edu [mailto:jacob at ldeo.columbia.edu] 
Sent: Wednesday, February 13, 2008 11:33 AM
To: Joe Tomasello
Cc: 'Arthur D Frankel'; ceus-earthquake-hazards at geohazards.usgs.gov
Subject: Re: [CEUS-earthquake-hazards] alternative hazard maps

A bit of history:

USGS produces computations and maps for several probabilities.
NIBS/SSC under FEMA funding developed the NEHRP guidelines, and its NEHRP
code
committee, and ground motion/geotechnical subcommittee voted to use the 2%
in
50 years as base for the nation-wide recommended NEHRP model code with the
very
important provisio that only 2/3 of the mapped values would go into the
design
basis.

This idea emerged at a national ground motion workshop organized by ATC in
California (I forgot when and where it was, probably in the early 90s). It
was
attended by more than 300 stakeholders/experts and it was essentialy the
battle
about a resolution whether probabilistic or deterministic hazards assessment
would be used for the basis of the the NEHRP model code.

The reasoning was: using 2/3 (based on some conservatism inherent in
buildings)
is allowed and would get the design values back to the ground motion values
in
high-hazards regions represented by the 10% in 50 years values. And the
Californians insisted on that. But they would also provide sufficient safety
for buildings  and structures in the low seismicity regions, where the 10%
in
50 years don't buy you much safety when the really damamging earthquekes
come
along that have recuurence periods often way beyond 475 years. And the low
seismicity folks insisted on that safety.

It was a smart compromise. If you really want to understand the physics and
details behind this argument, read the ground motion commentary chapters in
the
NEHRP documents; and familiarize yourselve with the steep slopes of hazards
curve for CA vs flat slopes of hazards curves in much of the CEUS.

Klaus Jacob
====================== Quoting Joe Tomasello <JT at reavesfirm.com>:

> Art, et al:
>
>
>
> 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?
>
>
>
>
>
> Joseph Tomasello, PE
>
> 5880 Ridge Bend Rd.
>
> Memphis, TN 38120
>
>
>
> Phone:
>
> (901) 761-2016 office
>
> (901) 821-4968 direct
>
> (901) 412-8217 mobile
>
> From: ceus-earthquake-hazards-bounces at geohazards.usgs.gov
> [mailto:ceus-earthquake-hazards-bounces at geohazards.usgs.gov] On Behalf Of
> Arthur D Frankel
> Sent: Wednesday, February 06, 2008 12:55 PM
> To: ceus-earthquake-hazards at geohazards.usgs.gov
> Subject: Re: [CEUS-earthquake-hazards] alternative hazard maps
>
>
>
>
> Seth,
>
>   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.
>
> 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.
>
> 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.
>
> 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.
>
> 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.
>
> 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).
>
> 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.
>
>
> Art Frankel
> U.S. Geological Survey
> MS 966, Box 25046
> DFC
> Denver, CO 80225
> phone: 303-273-8556
> fax: 303-273-8600
> email: afrankel at usgs.gov
>
>




----------------------------------------------------------------
This message was sent using IMP, the Internet Messaging Program.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Effect of PE Change .pdf
Type: application/pdf
Size: 13100 bytes
Desc: not available
Url : http://geohazards.usgs.gov/pipermail/ceus-earthquake-hazards/attachments/20080213/ea5c4653/attachment-0001.pdf