[PNW-Earthquake-Hazards] PNW Fault Working Group

Thu Nov 2 22:58:22 GMT 2006

I agree with Rus' proposed addition and parameters for the Stonewall
anticline.

On Mt. Angel, we were funded to trench it this fall, but when we went
out to perform detailed GPR and magnetic surveys over the fault traces
discussed by Rus as equivocal.  We were unable to image any shallow
deformation at the Dominic Road sites and abandoned our plans to trench
there.  At the Miller Rd site (where we had seismic lines of various
resolutions that imaged the fault) we found no GPR evidence for shallow
deformation, but did find a small ragged scarp? Less than a meter high
that was spatially associated with magnetic anomalies modeled by Rick
Blakely (see attached) as likely due to faulting.  We will be flying
LIDAR of the entire Mt. Angel fault this winter and so have decided to
defer the trenching till next fall.

________________________________

From: pnw-earthquake-hazards-bounces at geohazards.cr.usgs.gov
[mailto:pnw-earthquake-hazards-bounces at geohazards.cr.usgs.gov] On Behalf
Of Joan S Gomberg
Sent: Monday, October 30, 2006 8:39 AM
To: pnw-earthquake-hazards at geohazards.cr.usgs.gov
Subject: [PNW-Earthquake-Hazards] PNW Fault Working Group

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_wo
rkshop.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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