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Re: Near source factor for City of Newport, Oregon

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Friday, November 12, 1999

Curt and David,

PERHAPS  the reason both Na and Nv are equal to 1.0 in southern Oregon is
because the subduction zone is more than 15km off the coast for all of
southern Oregon.  The only area in California where it is nearer to the
coast than 15km is at Cape Mendocino (according to "Maps of Known Active
Fault Near-Source Zones in California and Adjacent Portions of Nevada",
published by ICBO, prepared by CA Division of Mines and Geology (CDMG),
February 1998).  I believe the subduction zone does not come nearer in
southern Oregon, but you'd better check for yourself.

To qualify as Type B (at a minimum), a fault must be believed capable of a
magnitude M6.5 or larger earthquake (regardless of slip rate),  -OR-  it
must have a slip rate greater than 2 mm per year (regardless of magnitude).
Type A faults have an even stricter requirement (must have a potential
magnitude >= M7.0  -AND-  a slip rate >= 5 mm per year).  See 97 UBC Table
16-U.

According to the above publication, the subduction zone (at least the
portion off the coast of California) is capable of a magnitude M8.3, and has
a slip rate of 35mm per year (I assume that means perpendicular to the fault
trace?).  This would make it (easily) a Type A fault.

No other Type A or Type B faults are shown crossing the border from CA to
OR.  Also, note that no maps were produced for Oregon and Washington because
there were no areas of Zone 4 in those states, not necessarily because there
were no Type A or B faults.

See also footnote 1 of 97 UBC Table 16-U.  This says that "Subduction
sources shall be evaluated on a site-specific basis."  Without further
guidance, I don't know what this means.  Perhaps some geotechs would
recommend a wider "near-source zone" based on the high expected magnitude of
the only known subduction zone in UBC-land.

You can contact the CDMG at 916-445-1923 for technical questions on this
publication.

Mark Swingle, SE
Oakland, CA

-------------------

From: "La Count, Curt" <Curt.LaCount(--nospam--at)Jacobs.com>
To: seaint(--nospam--at)seaint.org
Subject: FW: Near source factor for City of Newport, Oregon
Date: Friday, November 05, 1999

David,

Although Oregon has made parts of the coast zone 4, the Na and Nv factors
are set to 1.0.  I am not sure of the reasoning, but that's the code.

Curt La Count
Jacobs Engineering
Portland, OR

-------------------

From: David Hall
To: seaint(--nospam--at)seaint.org
Subject: Re: Near source factor for City of Newport, Oregon
Date: Thursday, November 04, 1999 9:55AM

Good question, I have a building that will be constructed in Brookings
Oregon, and I will have to find the same thing.

For those of you who do not know, Oregon adopted the 1997 UBC with
Amendments with one of those being that the coastal are of Southern Oregon
Be Zone 4.

I was going to contact SEAOC, but if anyone knows, I would also be
interested.

David A. Hall, S.E., P.E.
Senior Structural Engineer
David Evans and Associates, Inc.
2828 SW Corbett Ave
Portland, OR 97201
503-499-0280
Fax 503-223-2701
E-Mail dahl(--nospam--at)deainc.com