Free Republic
Browse · Search
News/Activism
Topics · Post Article

Skip to comments.

Shaken But Not Stirred: Rock Formations Reveal Past Quakes' Size Limit (So.Calif)
Science News ^ | 3-18-2006 | Sid Perkins

Posted on 03/19/2006 8:11:22 PM PST by blam

Shaken but Not Stirred: Rock formations reveal past quakes' size limit

Sid Perkins

Dozens of precariously balanced rocks in southern California tell a story just by standing there: Earthquakes that have occurred on nearby faults in recent millennia haven't exceeded magnitude 7. Researchers developing seismic-hazard maps for that and other areas are pondering how such rocky evidence might best be incorporated into their next round of updates.

STILL STANDING. Analyses of this 1.3-meter-tall stone southeast of Riverside, Calif., and other precariously balanced rocks suggest that recent earthquakes on nearby faults haven't exceeded magnitude 7. J. Brune, et al.

The Elsinore and San Jacinto fault zones lie about 35 kilometers apart and extend southeast from San Diego. In a 5-km-wide, 120-km-long swath centered between these faults, there are at least 60 rock formations that seem as if they'd topple with a modest jolt. Some of these stones are more than 2 meters tall, weigh a metric ton or more, and have been standing for many thousands of years, says Abdolrasool Anooshehpoor, a seismologist at the University of Nevada in Reno.

All the balancing stones are at least 14 km from the nearest fault, a hint that similar rocks closer to the faults have been toppled by past earthquakes, says Anooshehpoor. Field studies suggest that about 40 quakes of magnitude 6.5 or larger have occurred along fault zones in the region in the past 10,000 years, but the standing stones suggest that there's an upper limit to the size of those temblors.

Anooshehpoor and his colleagues estimated the stability of a dozen of the standing stones by analyzing their sizes and shapes. They found that, on average, ground motions with a side-to-side acceleration measuring about 30 percent of that of Earth's gravity would topple the rocks. A magnitude 7 or larger quake on those nearby faults would cause such vibrations, the researchers report in the March Geology.

Current estimates of seismic risk for the region suggest that ground motions large enough to topple the rocks occur, on average, every 2,500 years. However, the presence of so many precariously balanced rocks indicates that such motions are less frequent, says Anooshehpoor.

Previous estimates have included the possibility of large earthquakes occurring on previously unknown faults because other regions have been struck by such quakes, including the magnitude 6.7 quake that struck Northridge, Calif., in January 1994, says Mark D. Petersen, a seismologist at the U.S. Geological Survey in Golden, Colo. The information derived from the balanced rocks suggests that there are no active unknown faults near the San Jacinto and Elsinore faults.

--------------------------------------------------------------------------------

If you have a comment on this article that you would like considered for publication in Science News, send it to editors@sciencenews.org. Please include your name and location.


TOPICS: News/Current Events
KEYWORDS: formations; limit; past; quakes; rock; shaken; size; stirred

1 posted on 03/19/2006 8:11:31 PM PST by blam
[ Post Reply | Private Reply | View Replies]

To: blam
I live midway between the Elsinore and San Jacinto faults noted, six miles directly downstream from Diamond Valley Lake, a reservoir that holds more water than all the other reservoirs in southern California combined. I'm practicing treading water.
2 posted on 03/19/2006 8:24:06 PM PST by Hebrews 11:6 (Do you REALLY believe that (1) God is, and (2) God is good?)
[ Post Reply | Private Reply | To 1 | View Replies]

To: Hebrews 11:6

Fear not, FEMA will rise to the rescue. (That's the 2 buck chuck speaking).


3 posted on 03/20/2006 2:17:39 AM PST by poetknowit
[ Post Reply | Private Reply | To 2 | View Replies]

To: blam

On the other hand, maybe one of the reasons so many precipitously balanced rocks exist, is that they were previously much broader rocks and the larger earthquakes fractured and shook loose portions of their geologic constitution on their periphery.

i.e. ..then again they could be mistaken and the rocks are proof of more threatening quakes abounds throughout SoCA.

I don't see a very rigorous dismissal of orders of freedom, strength of materials, periodicity, and more importantly energy absorption by crack formation in these theories.


4 posted on 03/20/2006 2:25:03 AM PST by Cvengr
[ Post Reply | Private Reply | To 1 | View Replies]

Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.

Free Republic
Browse · Search
News/Activism
Topics · Post Article

FreeRepublic, LLC, PO BOX 9771, FRESNO, CA 93794
FreeRepublic.com is powered by software copyright 2000-2008 John Robinson