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le_bruce
climber
Oakland: what's not to love?
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Mar 31, 2009 - 02:57pm PT
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Anyone know if the slide debris reached the popular trail that extends from Mirror? Looks like it.
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simply theresa
climber
Yosemite
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Mar 31, 2009 - 03:29pm PT
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According to the Daily Report, the southern portion of the Mirror Lake loop trail is closed until further notice.
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GRJ
climber
Juneau AK
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Mar 31, 2009 - 03:47pm PT
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I hope they recycled the exploded timber in the middle of, what should be, the wilderness? Californians.... Drag that crap into camp four and let the monkeys have a bonfire!
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JEleazarian
Trad climber
Fresno CA
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Mar 31, 2009 - 04:08pm PT
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The rockfall was significantly north of the death slabs.
All of this makes me wonder, though, just what was so wrong with Camps 7 & 15 (aka the River Campgrounds) -- and whether we tax-paying family campers will ever get our Valley campsites back.
John
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yosguns
climber
San Francisco, CA
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Just got this uploaded. Aftershock rock fall from Sunday, 2 PMish.
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cleo
Social climber
Berkeley, CA
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whoa, cool aftershock photo!
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Mungeclimber
Trad climber
sorry, just posting out loud.
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images taken Sunday afternoon 3/29
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Argon
climber
North Bay, CA
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Great pictures! Greg - Was Ahwiyah Point known to be slide prone and were there any advance signs.
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drljefe
climber
Old Pueblo, AZ
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rumble, smash, bump
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sneville
climber
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Nice picture yosguns, did you get that from dinner ledge?
sean
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cleo
Social climber
Berkeley, CA
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SeismoBlog: http://seismo.berkeley.edu/blogs/seismoblog.php/2009/03/30/
Bursting Rocks and Trembling Earth
The seismic waves of Saturday's rock fall were recorded by many earthquake stations. The 33 depicted here are sorted top to bottom by increasing distance from Yosemite. It takes seismic waves longer to travel further distances, hence the "delay" of almost 60 seconds between the arrivals of the waves at the nearest and most distant stations. (Click for larger image.)
Screenshot of computer program showing seismic waves generated by the Yosemite rockfall, sorted by increasing distance.
While most of the Bay Area was rattled on Monday morning around 10:40 am by a magnitude 4.3 earthquake near Morgan Hill, another earthshaking event went almost unnoticed by the public - unless you were in Yosemite over the weekend. Early Saturday morning, a huge mass of rock came crashing down from Ahwiyah Point near Half Dome. Greg Stock, the Park Geologist at Yosemite, writes that the rocks "fell roughly 1800 feet to the floor of Tenaya Canyon, striking ledges along the way. Debris extended well out into Tenaya Canyon, knocking down hundreds of trees and burying the southern portion of the Mirror Lake loop trail... Fortunately, due to the event occurring in the early morning, there were no injuries."
But what happens when tons and tons of granite come crashing down unto the valley floor? Such an impact makes the ground vibrate and thereby creates seismic waves very similar to the ones being radiated by an earthquake. Indeed, on Saturday morning seismic stations all over Northern California and Nevada - as far away as 250 miles from Yosemite - registered these waves. The automatic earthquake location computer for Northern California at the offices of the USGS in Menlo Park picked up the recordings and calcuated an epicenter just half a mile to the northwest of Half Dome - which is actually pretty close to Ahwiyah Point. The program even computed a magnitude for the rock fall: Its impact had the same energy as a magnitude 2.4 earthquake.
While the seismic waves generated by a rock fall can be mistaken for the rumblings of an earthquake, the physics behind the two phenomena is completely different. Most earthquakes are the result of tectonic stress, which has accumulated in the rocks due to the movement of the lithospheric plates. A rock fall happens when the rock has been weakened by weathering. Water, which accumulates in cracks, freezes during the winter frosts. As ice occupies a larger volume as the same mass of liquid water, the freezing ice makes the rock expand and burst - similiar to a water bottle left in the freezer for too long. If such cycles of freezing and thawing are repeated often enough, the rock becomes loose and can break.
These rock bursts are by no means rare in granite world of Yosemite. Last October two rock falls hit some of the tents and cabins in Curry Village. In July 1996 more than 162,000 tons of rock cascaded down more than 2,000 feet, killing one visitor and crushing 500 trees. This blast was also recorded on many seismic stations, although it was somewhat smaller than Saturday's rock fall. After the 1996 event, BSL's Bob Uhrhammer analysed the seismic data carefully and reconstructed the details of the fall (see http://seismo.berkeley.edu/events_of_interest/yosemite/eoi_yos.html);
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corniss chopper
Mountain climber
san jose, ca
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noticed in the pictures that some of the runoff water is flowing on 'top' of the fresh rockfall debris. I would have thought
it would be quite porous and let the water inside the mass and so be invisible.
Suppose this means that enough of the rockfall was smashed to a fine powder to make a partially waterproof surface for the runoff to stay on the surface?
Interesting.
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Clint Cummins
Trad climber
SF Bay area, CA
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Photo of the release area from halfway up North Dome, June 2007:
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enjoimx
Big Wall climber
SLO Cal
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Reply to Dr. Sprock
1. You cant break the sound barrier unless some force beyond the force of gravity is involved. Rockfall doesnt break the sound barrier, unless someone attaches a rocket to the rock and blasts it downward.
2. A rockfal wont reach terminal velocity (usually) because it is sliding down the mountain, hence friction
3. Does a peice of lead fall faster than a feather in a non-air resistant environment? No. Neither does a large rock fall faster than a small rock.
Sure you're a Dr.?
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jsb
Trad climber
Bay area
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Does anybody know if this type of thing is usually seasonal... at least in Yosemite? (i.e. maybe melting ice or the thaw/freeze cycle is responsible for a lot of it?)
I know there was a lot of rockfall on Glacier Point Apron a long time ago... also near Rixon's Pinnacle. Were these in the springtime as well, by any chance?
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Dr.Sprock
Boulder climber
Sprocketville
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1) rocks do break the sound barrier
2)rocks can fall from the top of el cappy and never encounter wall friction
3)free falling rocks will have different impact velocities depending on shape, size and density
no, i am not a doctor, i'm a homo...
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Reilly
Mountain climber
Monrovia, CA
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"rocks do break the sound barrier"
I forgot it is April 1st.
Reminds me of the argument I had with somebody about raising the Kursk submarine from its watery grave. He maintained it didn't weigh 18,000 tons because it was underwater. Right, gravity doesn't work underwater, does it?
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hobo
climber
PDX
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Dr, what is wall friction?
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Dr.Sprock
Boulder climber
Sprocketville
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wall friction is what you get as your colon gets scraped by a wall burger not chased with enuff PBR.
Here is how you break the sound barrier with rocks:
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