[CEUS-earthquake-hazards] alternative hazard maps

[jacob at ldeo.columbia.edu]

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.