# RE: Seismic Recurrance Intervals

• To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
• Subject: RE: Seismic Recurrance Intervals
• From: "Cain, William" <bcain(--nospam--at)ebmud.com>
• Date: Wed, 5 Jul 2000 12:59:41 -0700
```Mike-
You want the 5% exceedance in 50 year event.  You can translate the various
recurrences back and forth with:

Probability of exceedance = (1-exp(-Lt))  where:
L = annual probability of exceedance  = mean probability of occurrence of 1
time in 1/L years
T = time interval for which you want the probability (e.g., 50 years

For example, if you want the 1000 year event, L= 1/1000=0.001
POE=(1-exp(-0.001*50)) = 0.049  (or approximately 5% exceedance in 50 year
interval)
POE=(1-exp(-0.001*100)) = 0.095 (or approximately 10% exceedance in 100
years)

What the actual motion goes to is a question for your friendly neighborhood
geotechnical engineer.  In California near Type A Faults, my understanding
is that the maximum considered earthquake is capped by what can be developed
deterministically on a fault (because the 500 year and 2500 year events do
not produce the 1.5 times relationship in the near source scenario).  In
most of the rest of the country, they use strictly the probabilistically
determined 2% in 2500 for the MCE and 2/3 of the MCE (which I believe is the
10% in 50 years (or 500 year event)).

Regards,
Bill Cain, S.E.
Oakland CA

-----Original Message-----
From:	Ritter, Mike [SMTP:mritter(--nospam--at)lgt.lg.com]
Sent:	Wednesday, July 05, 2000 11:36 AM
To:	SEAOC List Server (E-mail)
Subject:	Seismic Recurrance Intervals

Friends,

Another seismic question for the group.

I'm currently using the 2000 IBC on a project, which utilizes the
Maximum Considered Earthquake (MCE) maps.  No problem.  I can follow
the
methodology fine.  However, I have project criteria that states I'm
supposed to be using earthquake demand based on a 1000 year
recurrance
interval earthquake (or, I think, a 5% probability of exceedance in
50
years).  Most codes have seismic demand based on 10% probability of
exceedance in 50 years (or about 500 years).  I could not find
anything
listed like that for 2000 IBC, so I pulled out my FEMA 310 which
uses
the same (new) maps.  I found MCE defined as "2% probability of
exceedance in 50 years...".  That, I think, goes with a 2500 year
earthquake.  So.  How do I get to the 1000 year earthquake?

I understand the 2/3 factor in 2000 IBC (to get Sds and Sd1) does
something to take into account an approximate 50% reserve capacity
in a
typical code designed structure (i.e., Sds = Sms x 2/3 or Sds =
Sms/1.5)
but I'm not sure if it really reduces the recurrance interval.  If
it
does, what does it go to?  1000 years?  500 years?

Any help would be appreciated.

Michael D. Ritter, PE
Senior Structural Engineer
Lockwood Greene Technologies
1201 Oak Ridge Turnpike
Oak Ridge, TN  37831
(865) 220-4418
(865) 220-4310 (fax)

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