Subject: Re: Inglenook Date: Sun, 24 Jan 1999 12:45:37 -0800 From: Charles Watson <watson@seismo-watch.com> Organization: Advanced Geologic Exploration/Seismo-Watch To: "Pinpoint S.CA." <earth@Silicon.NET>, QUAKE-L Earthquake Discussion List <QUAKE-L@LISTSERV.NODAK.EDU> CC: PinpointEQ@listbot.com 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:watson@seismo-watch.com http://www.seismo-watch.com ______________________________________________________________________ To unsubscribe, write to PinpointEQ-unsubscribe@listbot.com Start Your Own FREE Email List at http://www.listbot.com/