Subject: Re: Inglenook
Date: Sun, 24 Jan 1999 12:45:37 -0800
From:  Charles Watson <[email protected]>
Organization:  Advanced Geologic Exploration/Seismo-Watch
To:  "Pinpoint S.CA." <[email protected]>, 
  QUAKE-L Earthquake Discussion List <[email protected]>
CC: [email protected]
References:  1



The geomorphology of the area in regards to today's M4.1 earthquake near
Ft. Bragg is interesting. I remember standing at the north side of the
Ten Mile Creek bridge abutment in the early 1980's and looking southeast
then northwest and wondered about why the rocks on the north bluff were
higher than those on the south. There were several visible remnant
Quaternary marine terraces above the Inglenook and Ft. Bragg but from my
vantage point at the bridge I could not see any on the north bluff.

So I dove up the road a few miles to an area called Kibersillah and
hiked up a short hill where I saw that the marine terraces were much
further up the ridge - nearly 150 ft. higher than at Inglenook Fen. I
noticed the topography of the area north and southeast of Ten Mile Creek
follows a west-northwest trend that has a much steeper relief. In
addition, Smith and Bucha Ridges, the two ridges on the north bluff,
extended westward from the Ten Mile Creek, forming an "unnatural bend"
in the shoreline expression. It was also clear to see the topography
south to Fort Bragg and beyond followed the San Andreas fault's
north-northwest structural trend.

It seemed to me then and in recall today that the Ten Mile Creek
drainage forms a unique expression of cross-cutting left-lateral
reverse-slip fault where the rocks on the north block are moving up and
to the northwest in relation to the south block and that this structure
was a possible "active fault" cutting remnant Quaternary (Holocene?)
marine terraces.

The UCB Seis lab's fault plane solution suggested west-northwest
trending reverse fault motion for today's M4.1 event which lends support
to the orientation and direction of this possible structural culprit. It
would be interesting to see if 1) this structure actually offset San
Andreas-type faults; and 2) if there is any geophysical anomalies
associated with the fault, and 3) if there were any open exposures of
the fault where one could see displacement.  I know of no studies of the 
neotectonics describing the Ten Mile Creek fault but would welcome
anyone's input.  I suspect one could look in the North Fork Ten Mile
Creek drainage's for possible fault exposures or map the North Fork
terraces in relation with those in the South Fork. Perhaps a good study
for students at Humboldt State University.

Oh yes and by the way, beautiful country. Hard to see the rocks through
the foliage, though. *grin*

--
---/----
Charles P. Watson, Consulting Geologist
Advanced Geologic Exploration/Seismo-Watch
P.O. Box 18012, Reno, Nevada 89511
Voice: 775-852-0992  /  Fax: 775-852-3226
mailto:[email protected]
https://seismo-watch.com

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