[CEUS-earthquake-hazards] alternative hazard maps

[Wang, Zhenming]

It is clear from these discussions that two fundamentally different things, earthquake and probabilistic ground motion (derived from PSHA), are being compared.

Here is a simple question for every one: If an M7.7 earthquake occurs every 500 years in the New Madrid seismic zone, how often will the ground motion generated by that earthquake be observed in Memphis, St. Louis, or Paducah?

Thanks.

Zhenming

________________________________
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 1:55 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
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