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tuolumne_tradster
Trad climber
Leading Edge of North American Plate
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Topic Author's Original Post - Jul 24, 2010 - 12:33pm PT
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General plate-tectonic map of the NE Pacific Ocean showing the North American, Pacific, Explorer, Juan de Fuca, and Gorda plates. The Juan de Fuca Ridge extends from West Valley to Cleft Segment, along the W margin of the Juan de Fuca plate. The seismic swarm discussed in this report lies within the darkened rectangle. The darkened circle shows the approximate location of the April 2001 Gorda Ridge seismic swarm. Courtesy of Bill Chadwick and NOAA.
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apogee
climber
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Jul 24, 2010 - 12:38pm PT
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Very interesting and very cool. Thanks for passing it along!
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Reilly
Mountain climber
The Other Monrovia- CA
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Jul 24, 2010 - 12:48pm PT
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Very illustrative, thanks! I take it the green dots in the second represent population?
One thing I've always wondered is 10 the theoretical max on the Richter scale?
Seems like for the Sierras to pop up out of the sea bed it would take something more.
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Tony Bird
climber
Northridge, CA
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Jul 24, 2010 - 12:55pm PT
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an open-ended logarthmic scale. a 9 is 10 times an 8. a 10 is 10 times a 9.
the good friday earthquake in alaska (1964) was possibly a 10.
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BooDawg
Social climber
Paradise Island
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Jul 24, 2010 - 01:06pm PT
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Great Illustrations of a geologically complex area! I love it that there are postings like this on S.T. Do you have one that shows the different terranes (small bits of micro-plates) that have drifted in from elsewhere and been melded or melted or fused to the W. edge of the N. American plate?
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Reilly
Mountain climber
The Other Monrovia- CA
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Jul 24, 2010 - 01:08pm PT
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Tony,
I know it is logarithmic but open-ended implies there can be greater than 10, right?
The AK quake has been determined to be 9.3 or 9.4 the last I checked.
My best friend there was in bed in Anchorage when it hit. It threw him
and his three bros onto the floor from their bunkbeds. For the ensuing
four minutes they couldn't even get to their knees to crawl out of their
bedroom! He described the scene as a bowl of goldfish dumped on the floor.
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Mighty Hiker
climber
Vancouver, B.C.
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Jul 24, 2010 - 01:35pm PT
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The geoglogists estimate that there is a magnitude 9 earthquake on the Juan de Fuca plate every 300 to 500 years. The last one was January 26th, 1700 - pinpointed by tsunami records from Japan. For more pictures and stuff: http://en.wikipedia.org/wiki/1700_Cascadia_earthquake
Such a quake would cause many deaths and enormous damage in southwest BC, western Washington, and western Oregon.
There probably was a person named Juan de Fuca, who claimed to have reached 47 degrees north on the west coast in 1592 (from New Spain/Mexico), but it's quite unclear whether he did. http://en.wikipedia.org/wiki/Juan_de_Fuca
(Francis Drake in the Golden Hind probably also didn't get so far north in 1578, despite claims from his boosters to the contrary. If he did get so far north, it led to no results.)
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Reilly
Mountain climber
The Other Monrovia- CA
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Jul 24, 2010 - 01:39pm PT
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Given Drake's larcenous nature it is hard to see what would have motivated
him to go so far north. Besides, where you gonna play at bowls along that coast?
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Tony Bird
climber
Northridge, CA
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Jul 24, 2010 - 01:43pm PT
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it's not the bo derek scale, more like the YDS. you got a better one, we can handle it. wait'll that asteroid finds its way in.
my bro moved to alaska, fortunately after 1964. but there are plenty of anecdotes like that. full grown pine trees in the city park whipping back and forth all the way to the ground. water coming into the fjords hundreds of feet above sea level, people scrambling to get higher, not all successful. on good friday a lot of people thought it was the end of the world.
btw, i believe the richter scale is maximum intensity. there's also something called seismic moment. think YDS versus grade III, IV, etc., but i could be wrong about that. time for a real know-it-all to speak up.
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Thorgon
Big Wall climber
Sedro Woolley, WA
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Jul 24, 2010 - 02:03pm PT
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Geology Bump! Didn't Baker blast some steam about 1980? We would like to think it is dormant! LOL
Thor
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tuolumne_tradster
Trad climber
Leading Edge of North American Plate
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Topic Author's Reply - Jul 24, 2010 - 02:30pm PT
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from Wikipedia...
The Juan de Fuca Plate, named after the explorer of the same name, is a tectonic plate, generated from the Juan de Fuca Ridge, and subducting under the northerly portion of the western side of the North American Plate at the Cascadia subduction zone.
The smallest of Earth's tectonic plates, the Juan de Fuca Plate is a remnant part of the once-vast Farallon Plate, which is now largely subducted under the North American Plate.
here is an image from a 3D USGS model showing the depth in km to the top of the subducting J de F plate and earthquakes
http://pubs.usgs.gov/ds/91/
Here's the text that goes with these figures
We present an updated model of the Juan de Fuca slab beneath southern British Columbia, Washington, Oregon, and northern California, and use this model to separate earthquakes occurring above and below the slab surface. The model is based on depth contours previously published by Flück and others (1997). Our model attempts to rectify a number of shortcomings in the original model and to update it with new work. The most significant improvements include (1) a gridded slab surface in geo-referenced (ArcGIS) format, (2) continuation of the slab surface to its full northern and southern edges, (3) extension of the slab surface from 50-km depth down to 110-km beneath the Cascade arc volcanoes, and (4) revision of the slab shape based on new seismic-reflection and seismic-refraction studies. We have used this surface to sort earthquakes and present some general observations and interpretations of seismicity patterns revealed by our analysis. In addition, we provide files of earthquakes above and below the slab surface and a 3-D animation or fly-through showing a shaded-relief map with plate boundaries, the slab surface, and hypocenters for use as a visualization tool.
this one shows a cross section
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TGT
Social climber
So Cal
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Jul 24, 2010 - 03:22pm PT
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Hm,
Never realized that the JDF plate was a true independent plate with a spreading zone on the west side.
Wonder if it's going to produce a volcanic island arc some day.
Plenty of precedent on the west coast. That's largely what makes Calif Geology so interesting.
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nature
climber
Tucson, AZ
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Jul 24, 2010 - 03:28pm PT
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I'm particular to the Gorda plate myself, Juan's little brother.
I was in Humboldt when the three ~7.0's hit in 1991.
Doing graduate work there in Geology was a pretty bad ass experience. Gary Carver is a madman and a genius. A fair amount of what we know about Cascadia is attributed to his work (both along the north coast and Alaska).
Baker blasted steam more recently than that. 1981 St. helen's blew. I lived in Tacoma at the time. Saw the mushroom cloud out my back window. Watch TV all day long when the massive eruption occurred. Sure am glad the winds were blowing the other way.
One thing I've always wondered is 10 the theoretical max on the Richter scale?
Seems like for the Sierras to pop up out of the sea bed it would take something more.
A lot of little events went into jacking the Sierra Nevada microplate up 10,000+ feet. if it went all at once the Richter scale would be meaningless.
Next time you drive HWY 395 look at the eastern escarpment of the Sierra and think of it as being a 10,000 foot high normal fault.
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Reilly
Mountain climber
The Other Monrovia- CA
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Jul 24, 2010 - 04:10pm PT
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Mt Baker in more like '75 ...
It sounded waaay scary!
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tuolumne_tradster
Trad climber
Leading Edge of North American Plate
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Topic Author's Reply - Jul 24, 2010 - 05:11pm PT
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Mt St Helens May 17 09:00
Mt St Helens May 18 08:35
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Jaybro
Social climber
Wolf City, Wyoming
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Jul 24, 2010 - 05:22pm PT
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Nature, all genius' are madmen/women, just the price of admission.
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tuolumne_tradster
Trad climber
Leading Edge of North American Plate
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Topic Author's Reply - Jul 24, 2010 - 05:36pm PT
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Developed in 1935 by Charles Richter in collaboration with Beno Gutenberg, both of the California Institute of Technology, the Richter scale was originally intended to be used only in a particular study area in California, and on seismograms recorded on a particular instrument, the Wood-Anderson torsion seismometer.
Richter arbitrarily chose a magnitude 0 event to be an earthquake that would show a maximum combined horizontal displacement of 1 micrometre on a seismogram recorded using a Wood-Anderson torsion seismometer located 100 km from the earthquake epicenter. This choice was made to prevent negative magnitudes from being assigned. However, the Richter scale has no upper or lower limit. Sensitive modern seismographs now routinely record quakes with negative magnitudes.
Because of the limitations of the Wood-Anderson torsion seismometer used to develop the scale, the original ML cannot be calculated for events larger than about 6.8. Many investigators have proposed extensions to the local magnitude scale, the most popular being the surface wave magnitude MS and the body wave magnitude Mb.
REF: http://www.wordiq.com/definition/Richter_magnitude
The Richter scale has been largely replaced by the Moment magnitude scale (MMS) to estimate magnitudes for all modern large earthquakes by the United States Geological Survey. The MMS is used by seismologists to measure the size of earthquakes in terms of the energy released. The magnitude is based on the moment of the earthquake, which is equal to the rigidity of the Earth multiplied by the average amount of slip on the fault and the size of the area that slipped. The scale was developed in the 1970s to succeed the 1930s-era Richter magnitude scale (ML). Even though the formulae are different, the new scale retains the familiar continuum of magnitude values defined by the older one.
The energy released by nuclear weapons is traditionally expressed in terms of the energy stored in a kiloton or megaton of the conventional explosive trinitrotoluene (TNT). A rule of thumb equivalence from seismology used in the study of nuclear proliferation asserts that a one kiloton nuclear explosion creates a seismic signal with a magnitude of approximately 4.0.
REF: http://en.wikipedia.org/wiki/Moment_magnitude_scale
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Hardly Visible
Social climber
Llatikcuf WA
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Jul 24, 2010 - 05:58pm PT
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Boodawg,
You may have to zoom in to about 200% to read these but here's a sampling of accreted terrains in the north cascades.
I'm pretty sure everything cretaceous or older drifted in from somewhere else.
Then of coarse a little furter north and west you have Vancouver and The Queen Charlotte Islands most of which is now thought to have come from equatorial latitudes.
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tuolumne_tradster
Trad climber
Leading Edge of North American Plate
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Topic Author's Reply - Jul 24, 2010 - 06:24pm PT
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Hardly Visible: you are referring to "Wrangellia" (named after the Wrangell Mountains in Alaska) shown here in this Late Cretaceous Paleogeographic reconstruction from Paleogeography Google Earth. Wrangellia is the terrain just west of the 2 large land masses, the Canadian Shield and Eastern N America...this is before Wrangellia collided and accreted on to N America forming what is now Western North America. The yellow outline showing the present location of N & S America is there for reference.
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tuolumne_tradster
Trad climber
Leading Edge of North American Plate
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Topic Author's Reply - Jul 24, 2010 - 07:40pm PT
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Then of coarse a little further north and west you have Vancouver and The Queen Charlotte Islands most of which is now thought to have come from equatorial latitudes.
Yes this is based on the remnant paleomagetic vector preserved in volcanic rocks & sediments. Here is a recent paper that suggests these terrains may not have originated so far south...
Late Cretaceous paleogeography of Wrangellia: Paleomagnetism of the MacColl Ridge Formation, southern Alaska, revisited
John A. Stamatakos1 Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, San Antonio, Texas 78238, USA
Jeffrey M. Trop Department of Geology, Bucknell University, Lewisburg, Pennsylvania 17837, USA
Kenneth D. Ridgway Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907, USA
GEOLOGY August, 2010
Volcanic and sedimentary strata of the Late Cretaceous MacColl Ridge Formation were sampled and demagnetized to reevaluate the paleomagnetically derived paleolatitude of the allochthonous Wrangellia terrane. Characteristic directions from 15 sites representing ∼750 m of the MacColl Ridge Formation (80 Ma) reveal a reversed-polarity primary magnetization yielding a paleomagnetic pole at 126°E, 68°N, A95 = 9°. Comparison of this pole with the Late Cretaceous reference pole for North America indicates 15° ± 8° of latitudinal displacement (northward) and 33° ± 25° of counterclockwise rotation. In contrast to previously reported low paleolatitudes (32° ± 9°N) for the MacColl Ridge Formation, these new results place the Wrangellia terrane at a moderate paleolatitude (53° ± 8°N) in the Late Cretaceous.
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