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RE: seismic forces

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It all happened on a dark and rainy night. 
The BSSC NEHRP Provisions and NEHRP Commentary tell the story much better than I can.  In a nut shell, the infrequent large historic earthquakes did not show up to properly characterize a uniform risk.  The probability was adjusted to capture the large infrequent events by going to 2% excedance in 50 years as opposed to the more familiar 10% chance of excedance in 50 years and yielding a uniform hazard.  This all happened in the late 1990's and was vetted with some incredibly bright engineers and seismologists.  FEMA knew there was a problem, and the USGS zeroed in on it. 
This probability shift made little difference in the classic California models, but made a huge difference in the Eastern North American seismic events.  With all of that, there have been some recent revelations that indicate that the ENA events may be much more regular in time as evidenced in trenching studies.  The implications are that a time dependent model may be more appropriate for ENA events.  For example the NMSZ big one is actually 3 events occurring in less than a year but the triplets occur about every 400 years.  This could drive the magnitude down in ENA events.  But historically, these events were enormous and effected huge areas of the US because of the attenuation differences between ENA and Pacific rim earthquakes.  In the NMSZ, between December 1811 and May 1812, there were 3 earthquakes that were larger than the 1906 San Francisco earthquake, and there were about 2,000 event recorded in Nashville, TN which is quite a distance from the perceived epicenter.  There was damage recorded in Charleston and church bells rang in Boston.  Now THAT is an earthquake. 
There are some changes coming down the pike, but they are not here yet.  You can follow the trends by watching what comes up on  USGS is very methodically vetting the changes. 

Regards, Harold Sprague

From: sgordin(--nospam--at)
To: seaint(--nospam--at)
Subject: seismic forces
Date: Mon, 24 Nov 2008 12:19:26 -0800

Based on the 2006 IBC (ASCE07-05), the seismic accelerations for Hayward, CA (right on the Hayward fault) appear to be about 60% lower than those for Sikestone, MO, and about the same as for Summerville, SC. 
Historic (1812 and 1886) considerations aside, any comments?
V. Steve Gordin, SE
Irvine CA

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