Portland earthquake: 'Chance of a mag...

Portland earthquake: 'Chance of a magnitude 9 quake' warn Oregon experts

There are 1 comment on the Examiner.com story from Apr 8, 2014, titled Portland earthquake: 'Chance of a magnitude 9 quake' warn Oregon experts. In it, Examiner.com reports that:

Portland's earthquake on Sunday was a 3.3-magnitude earthquake that was felt throughout the Portland area around 8:30 p.m. While there were no major damages or injuries after the quake, it was a wake-up call for Oregon residents to be prepared for a much bigger earthquake along the Cascadia subduction zone.

Join the discussion below, or Read more at Examiner.com.

“Geologist [I'm Climate Change]”

Since: Mar 07

formerly Nuneaton

#1 Apr 9, 2014
Interesting article.

It forgot to mention the plate convergence vector of the plates along Cascadia.

The N. American plate obviously converges on the Juan de fuca rift zone at ~4.5cm/yr or thereabouts.

The Juan de fuca plate however is ALSO moving and has a SE vector of spread convergent fastest @ California & S. Oregon.

Also note that the SW. part of the basin & range is spreading via the gulf of California rift which is attempting & beginning to succeed in reaching the Yellowstone hotspot. The vector of spread is NW.

Add to this the spreading of the Basin & range as a result of outflow from the hotspots @ Raton-Clayton & Colorado which forces spreading WNW.

Do not forget the outflow from the Yellostone hotspot itself (which has more emphasis on "hot" rather than outflow), and forces slower spreading then the other hotspots WSW.

There is also the spreading from the Chilcotin hotspot (Wells Grey-Clearwater volcanic field) in Canada which has already had its spreading measured @ Seattle & Vancouver as it is SW the same as the N. American plate & likely adds less then 1cm/yr to the total of ~4.5cm/yr toward the Juan de Fuca rift.

Result is that convergence is fastest in S.Cascadia, in the Lassen area, and decreases N. along Cascadia to become slowest in Central Vancouver island. The convergence rate also slows along the Gorda arc from Lassen to mendocino (clear lake), as a result of the rate of spread of the Gorda plate (subducting) being slowest in the south.

A magnitude 9 is probable at the percentage point listed in the article, but its probability of reaching as far as Seattle is near zero & is likely to begin @ the Gorda arc near Mendocino & terminate in either S. Oregon or central Oregon, with the only vector possible after the crustal relaxation of 1700AD(WNW) being NW which remains compressed @ Mendocino, having been bypassed by the 1700AD event which merely jogged the forearc sideways.
For tyhe record, the 1700AD event appears to have begun near Lassen in the junction box with the Gorda arc WNW vector slicing the Gorda forearc, and jogging it in front of Mendocino. The fault break then spread from 1st break N along Cascadia with relaxation of the crust @ WNW vector with the largest runout in central Oregon at about 30m or so. The WNW vector runout then slowed & petered out in the seattle area in the Junction box with the Garibaldi arc. The crust weakened by a recently slipped fault then moved in a WSW vector from Seattle onward and relaxed with an arc runout WSW. from approx seattle through the gribaldi arc to central vancouver island with the runout decreasing W. and also relaxed with arc runout from Seattle area through to S. Washington re using the prior fault a few minutes old.

Have a nice day: Ag

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