[CEUS-earthquake-hazards] reply to Ellis Krinitzsky
Arthur D Frankel
afrankel at usgs.gov
Fri Feb 22 11:28:13 MST 2008
Ellis,
I don't think the example you cite is applicable to the case of the
logic tree we use for New Madrid magnitudes. In our logic tree we are
trying to account for the uncertainty in the magnitude of characteristic
(1811-12 type) earthquakes in the New Madrid source zone.
In your example with two faults, a probabilistic hazard assessment would
use the range of magnitudes specific for each fault. It does not average
the magnitude between the two faults.
In your earlier email, you expressed doubt about determining the rates
of large earthquakes by extrapolating the rates of smaller earthquakes
using a "b-line."
This is not what is done for the New Madrid characteristic earthquakes
(1811-12 type earthquakes). The average recurrence rate for the 1811-12
type earthquakes is determined from the dating of sand blows (see Tuttle
et al., 2002 BSSA), which shows that previous such events occurred around
1450 and 900 A.D.
-Art
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
"Krinitzsky, Ellis L ERDC-GSL-MS Emeritus"
<Ellis.L.Krinitzsky at usace.army.mil>
02/22/2008 08:40 AM
To
"Arthur D Frankel" <afrankel at usgs.gov>, "Wang, Zhenming"
<zmwang at email.uky.edu>
cc
<ceus-earthquake-hazards at geohazards.usgs.gov>
Subject
RE: [CEUS-earthquake-hazards] FW: reply to Joe Tomasello; buildings codes
and earthquake hazard
Art,
I saw elsewhere you came to your values using a logic tree.
Suppose you had two faults, one with a potential for M6, another for M8.
If
you average them you have an M7. If you designed for that M7 and you had
an
M8, you would be under designed. Yet the logic tree makes you do exactly
that. Plus other illogical moves.
You need to solve those problems first.
Ellis
-----Original Message-----
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: Tuesday, February 19, 2008 10:01 PM
To: Wang, Zhenming
Cc: ceus-earthquake-hazards at geohazards.usgs.gov
Subject: Re: [CEUS-earthquake-hazards] FW: reply to Joe Tomasello;
buildings
codes and earthquake hazard
Zhenming,
These two statements are not contradictory, when taken in the context
that
I wrote them.
In the quote from my response to my comment, I was referring to the
ground
motions observed at any given location over time. At any particular site
the
ground motions with a 2% chance of being exceeded in 50 years, will occur,
on
average, once in 2500 years. As I said before, another way to express this
is
that these ground motions have a 1/2500 chance of being exceed each year.
The point of my recent email is that each time an 1811-12 type
earthquake
occurs, there will be some locations that will experience the 2%/50 ground
motions or larger, because of the spatial variability of ground motions.
The
set of sites that experience these higher ground motions will likely be
different for each occurrence of this type of earthquake, because of the
variability of ground motions from earthquake to earthquake. This apparent
temporal variability of ground motions will occur even for successive
earthquakes on the same fault, because of differences in rupture
propagation
and slip on the fault from earthquake to earthquake.
As you design buildings to ground motions with lower probability levels,
you
are protecting a larger fraction of buildings from the range of ground
motions expected during the next 1811-12 type earthquake.
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
-----ceus-earthquake-hazards-bounces at geohazards.usgs.gov wrote: -----
To: "ceus-earthquake-hazards at geohazards.usgs.gov"
<ceus-earthquake-hazards at geohazards.usgs.gov>
From: "Wang, Zhenming" <zmwang at email.uky.edu>
Sent by:
ceus-earthquake-hazards-bounces at geohazards.usgs.gov
Date: 02/19/2008 07:34AM
Subject: [CEUS-earthquake-hazards] FW: reply to Joe
Tomasello;
buildings codes and earthquake hazard
Here is another inconsistent statement on the national
seismic hazard
maps.
" You have the mistaken notion that 2%/50 values are only
observed
once in 2500 years. This is flat out wrong. "
In a response to our comment (Wang and others, 2005) that
was
published on Seismological Research Letter (Frankel, 2005), the
interpretation was "the ground motion with 2% PE in 50 years is exceeded
once, on average over 2,500 years, so that it has a 1/2500 annual
probability
of being exceeded."
As demonstrated earlier, for a single M7.7 earthquake
with 500 year
recurrence interval in the New Madrid seismic zone, ground motion with
2,500-year return period (2% PE in 50 years) means there is about 20
percent
probability that ground motion will be exceeded if the M7.7 earthquake
occurs. In other words, if the ground motion with 2,500-year return period
is
selected for engineering design, we has a confidence level of 80% (not
being
exceeded) if the M7.7 earthquake occurs.
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: Friday, February 15, 2008 1:50 PM
To: Joe Tomasello
Cc: ceus-earthquake-hazards at geohazards.usgs.gov;
mpetersen at usgs.gov
Subject: Re: [CEUS-earthquake-hazards] reply to Joe
Tomasello;
buildings codes and earthquake hazard
Joe,
I feel I need to reply to you on the bulletin board,
since you seem
to want to keep this discussion going and you directly ask me questions.
My
apologies to Paul Segal and others who are fed up with this.
I guess you missed one of the key points of my previous
email. I
said that the IBC values (2/3 times the ground motions with 2% probability
of
exceedance in 50 years [2%/50]) were probably experienced inMemphis during
the 1811-12 earthquakes, based on intensity data from those earthquakes.
You have this mistaken notion that IBC values (2/3
times the 2%/50
motions) are only observed once in a millenium. You have the mistaken
notion that 2%/50 values are only observed once in 2500 years. This is
flat
out wrong. Some locations will experience 2/3 times the 2%/50 ground
motions during the next 1811-12 type earthquake. Some locations will
experience the 2%/50 ground motions during the next 1811-12 type
earthquake.
This is a simple consequence of the observed variability of earthquake
ground
motions.
As far as the magnitude issue you bring up, the USGS
uses the
range of magnitudes that various seismologists have determined for the
1811-12 earthquakes from the intensity data. The central values of these
moment magnitude determinations range from 7.4-7.5 (Hough et al., 2000 in
JGR) to 7.8 (Bakun and Hopper, 2004 in BSSA) to 8.0-8.1 (Johnston, 1996
in
Geophysical Journal). In the national maps we use a logic tree to express
this range, with a value of 7.7 given the highest weight. I used M7.7 in
the scenario in my previous email, because it is in the center of the
range
of magnitudes determined for the 1811-12 earthquakes. I also gave results
for a M7.4 earthquake.
In the example in my previous email, I placed the
scenario
earthquake where the current seismicity trend for the New Madrid seismic
zone
is located. The closest distance to downtown Memphis is about 60 km.
-Art
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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