[PNW-Earthquake-Hazards] PNW Fault Working Group email from Tim Walsh

[Joan S Gomberg]

Hi Art,
Sorry I missed your deadline. We just changed from a failing e-mail server 
to a new one, changed from Groupwise to Outlook, and changed our e-mail 
addresses. Many things have slipped throught the cracks. The main item for 
me to comment on is the treatment of the faults in the Yakima fold belt. I 
agree with Bob that the treatment of including only those faults with 
demonstrated Holocene activity is misleading in two ways. One, as Bob 
pointed out, if the recurrence interval is longer than the length of the 
Holocene then faults that have not had a Holocene event may be more likely 
to have one than faults with Holocene activity. I realize, following Rus? 
argument , that it would be difficult to accommodate that. The other 
problem, though, is that most of them haven?t been investigated, i.e., 
high epistemic uncertainty. This could lead to the issue that the building 
officials have, which is that as paleoseismology investigations are done 
the regional hazard may change dramatically as new trenches are dug. I 
wonder if this might be best treated as a source region using the net 
shortening rate for the fold belt as a whole rather than concentrating on 
those structures that have been studied in some detail.
Regards,
Tim

Joan Gomberg
US Geological Survey
University of Washington
Dept of Earth and Space Sciences, Box 351310
Seattle, Washington 98195-1310
206-616-5581(office) 
206-553-8350 (fax)
206-941-7498 (cell)
gomberg at usgs.gov
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