[PNW-Earthquake-Hazards] PNW Fault Working Group
Joan S Gomberg
gomberg at usgs.gov
Mon Oct 30 09:39:05 MST 2006
Thank you all for agreeing to participate in the PNW Fault Working Group.
I have 'volunteered' to facilitate the business of the group with the
primary goal of helping it to come to informed decisions that can be
passed on to the national seismic hazard mapping project. Secondarily, I
hope this will be educational for all involved. For those of you I have
not met or only recently have met, my principal qualification to do this
is that I begin from a state of nearly complete ignorance (and geologic
expertise) and thus, can guarantee that I have absolutely no biases! If
it?s okay with all of you, I?d like to try having our electronic exchanges
using a listserv, which is basically just a moderated email discussion
group with the benefit of keeping an online archive. As this listserv has
already been set up, but was as yet unused, I thought this might be a
great way to both begin our discussions and try out this tool (possibly
for use more broadly). I?ve taken the liberty of subscribing all of you
to the listserv and have sent this email through it; for more information
about it please see the webpage at
http://geohazards.cr.usgs.gov/mailman/listinfo/pnw-earthquake-hazards.
Please note that although described as for more general purposes,
currently only the Working Group is subscribed to it, so no one else will
receive our emails or be able to look at the archive (although I see no
reason why ultimately we shouldn?t use it as a means of helping to
document the basis of any recommendations). If you think this is not a
good idea, please don?t hesitate to let me know. If not, please send all
email, attachments, etc. to pnw-earthquake-hazards at geohazards.cr.usgs.gov,
and after I screen them (as the listserv moderator) the entire group will
receive them.
As you?ve all received Art?s emails, there?s no need to repeat the PNW
Fault Working Group?s current charge. 4 additions and 4 modifications to
existing fault parameters are being considered . These are listed below
with a few comments about background information with which to evaluate
each. We hope that the person suggesting the change and/or who is an
obvious source of information about the particular fault will provide the
group with additional information. I have attached the comments of Bob
Yeats and responses from Rus Wheeler that you have already been sent again
for convenience. An overview of what fault parameters are currently used,
why, etc. is in the presentation under "Introduction: Overview of
scientific issues: why are we here?" by Frankel made at the National Map
workshop held this Spring. Most of the presentations from the workshop
and referenced below can be found at
http://earthquake.usgs.gov/research/hazmaps/whats_new/workshops/pacNW_workshop.php.
Please also note that Art emphasizes that when thinking about deciding
whether changes should be made, one should consider the policy of the
National Maps that only faults where a slip rate or recurrence time has
been estimated or determined from measurements on that fault can be used
(e.g. slip rates based entirely on analogs are not sufficient). Please
provide your contributions, using the listserv, before the end of November
(the earlier the better). When enough information has been exchanged I
will try to schedule a conference call, or if it appears necessary, a
face-to-face meeting. Please keep in mind the goal of providing final
recommendations my mid-January.
Thanks very much!
Joan Gomberg
*** Changes to be Considered ***
New additions:
The Stonewall anticline (Newport, OR)
This has been suggested by Bob Yeats and is discussed by he and Rus
Wheeler in the attachments.
More faults in Yakima fold belt
This has been suggested by Bob Yeats and is discussed by he and Rus
Wheeler in the attachments.
Boulder Creek fault (has multiple events with age constraints)
Brian Sherrod has new information to share.
Little River fault (has multiple events with age constraints)
See presentation under "Faults in the Puget Sound region and elsewherein
western Washington" by Alan Nelson.
Slip rate changes:
Portland Hills fault
Change slip rate from 0.1 mm/yr (in 2002 maps) to 0.07 mm/yr; proposed by
Ian Madin
Bolton fault
Change slip rate from 0.013 mm/yr (in 2002 maps) to 0.015 mm/yr; proposed
by Ian Madin
Mt Angel fault
There is new information, noted by Bob Yeats and and discussed by he and
Rus Wheeler in the attachments, but it is not clear that any change is
being advocated in the treatment of this fault.
SWIF
Change fault geometry and dimensions; see Art?s Introductory presentation
that notes a change dip from 60 to 45 degrees,seismogenic layer thickness
from 17.3 to 20 km, length from 63 to 86 km. Assuming the same uplift
rate of 0.6 mm/yr this changes the characteristic magnitude from M7.2 to
M7.3 with Tchar from 3100 to 1700 yr, and for a M6.5 the recurrence time
changes from 930 to 400 yr. For 0.5 mm/yr strike slip component (derived
assuming pure north-south convergence): Tchar=2900 yr and the recurrence
time for a M 6.5 becomes 680 yr. Combining these equally (1/2 reverse and
1/2 strike-slip motion) results in Tchar= 1300 yr and a M 6.5 recurrence
interval of 310 yr, which are much shorter times than used for the Seattle
fault.
Also see presentations under "South Whidbey Island Fault experience" by
Rick Blakely
Joan Gomberg
US Geological Survey
University of Washington
Dept of Earth and Space Sciences
Box 351310
Seattle, Washington 98195-1310
206-616-5581
gomberg at usgs.gov
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