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# RE: USGS vs NEHRP

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: USGS vs NEHRP
• From: "Sherman, William" <ShermanWC(--nospam--at)cdm.com>
• Date: Thu, 15 Sep 2005 22:32:03 -0400

```Although I had previously looked at the 2003 NEHRP Commentary, I took
another look, at your suggestion.  I did find some paragraphs that
appear to be related - but they seem to be written in "doublespeak"!

Pages 318-319 explain that deterministic earthquakes are used in coastal
California in lieu of probabilistic earthquakes.  That I understood.
Then pages 322-324 explain in greater detail how mapping was developed
for "Regions of High Seismicity Near Known Fault Sources".  It refers to
"USGS deterministic hazard maps" - but I cannot find such maps on the
USGS seismic hazard website.  However, the two portions I really have
trouble making sense out of are as follows:

1. Regarding MCE ground motion values and with a short period "1.5 *
ceiling value" of 1.50g: "When the 1.5 times the ceiling values are
reached, then they will be used until the deterministic maximum
considered earthquake map values of 1.5g (long period) and 0.60g (short
period) are obtained.  From there, the deterministic maximum considered
earthquake ground motion map values will be used."

2. It then states that in "some cases" where probabilistic map values
exceed the 1.5 * ceiling values but are less than the deterministic
values, "the probabilistic map values will be used for the maximum
considered earthquake ground motions".

Regarding item 1: first, I think the "long period" and "short period"
values are reversed; but more importantly, it is not clear whether the
deterministic values are used when they are "greater than" or "less
than" the 1.5 * ceiling values?

Initially, "greater than" seems to make sense - but if deterministic
earthquakes are used to reduce design values relative to probabilistic
values, perhaps "less than" makes sense? However, if "less than" applies
in item 1, then deterministic values would be used since they must be
less than the probabilistic values - but that would negate item 2.
Therefore, "greater than" must apply in item 1, and item 2 only applies
when deterministic values are greater than both 1.5 * ceiling values and
greater than probabilistic values.

Couldn't they just say that "the lesser of the deterministic value and
the probabilistic value is used when both are greater than 1.5 * the
ceiling value of 1.5g"?

This also seems to contradict my previous understanding that
deterministic values apply when they are less than probabilistic values.
Apparently that is not true when deterministic values are less than the
1.5 * ceiling values; apparently, probabilistic values are then used.

But for my site, I'm not sure this answers my question.  Perhaps NEHRP
reduced the design value to 1.5g since the probabilistic value must have
been less than the deterministic value; and the probabilistic value must
be equal to 1.5g - otherwise a lower value would have been shown.  But
the USGS map value still does not make sense if it is based on
probabilistic values (per my understanding from NEHRP Appendix B); it
would only make sense if the USGS value is based on the deterministic
value?

(Your grade is "A+" if you understand what I just wrote!)

William C. Sherman, PE
(Bill Sherman)
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com

-----Original Message-----
From: Harold Sprague [mailto:spraguehope(--nospam--at)hotmail.com]
Sent: Thursday, September 15, 2005 4:50 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: USGS vs NEHRP

Bill,
There is discussion on this in the NEHRP commentary.  I am working 12-16
hours a day, 7 days a week in Biloxi, MS right now or I would look it
up.

Regards,
Harold Sprague

>
>
>For a given latitude and longitude in California, the 0.2 second MCE
>acceleration is 1.50 per the NEHRP/ IBC/ ASCE 7 maps (using the
>CD-ROM); but the value returned by the USGS web site is 2.00.  Why is
>the USGS value 33% higher?
>
>The documentation for NEHRP says that they modified some contours
>downward in California relative to USGS probabilistic maps based on
>deterministic seismic data.  But my geotechnical engineer is coming up
>with the same ground accelerations as the USGS web site, based on
>deterministic methods for known faults.  Thus, the deterministic method

>does not seem to explain this difference?
>
>
>William C. Sherman, PE
>(Bill Sherman)

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