Local Overview
Posted on 06/27/2005 3:28:20 PM PDT by sofaman
Magnitude 4.0 - local magnitude (ML) Time Monday, June 27, 2005 at 3:17:33 PM (PDT) Monday, June 27, 2005 at 22:17:33 (UTC)
This one was right near a 4.9 that occured 11 days ago. I agree, most likely an aftershock.
Would not the fear of the proverbial "Big One" be greater if we were not getting mild ones in between stronger jolts?
Do not mild earthquakes indicate normal movement (i.e. relieving of stress)?
If California had gone 20 years or so without any earthquakes, I would feel that the "Big One" was much more imminent.
From the maps I've looked at, this one appears to be a little bit west of the San Andreas.
Earthquakes of this size relieve basically no stress whatsoever.
It would take ONE THOUSAND quakes of this size (M 4.0) to relieve the same amount of stress as an M 6 quake.
Every earthquake on a fault line relieves stress.
Actually, some increase it.
Point is anything under M6 is basically meaningless in the grand scheme of things in "stress release."
The overwhelming % of the stress in Southern California (or the world, for that matter) is released in quakes of M6 or higher.
Only significant in its proximity to so many others.
Actually it sounds like you are confirming what I stated, rather than disputing it. Consider that the map in post seven shows 386 earthquakes occurring in California in the last week. So how many is that over a period of time?
At that rate that is about 20,000 earthquakes in a year. Given that rate may be unusually high, but even if it is off by a factor of 2, that is still 10,000 quakes in a year.
As I said, I would be more concerned of a big one if we were not having any small ones.
Big quakes relieve more stress than smaller quakes. No shocker there.
Hey, Orion...what's the town in the overview?
I was at a public meeting in Yucaipa giving earthquake advice and their reasoning was that the fault lines above and near the San Andreas could trigger a major earthquake if they started have earthquakes on what has essentially been dormant faults. They sit right over the San Andreas. He was explaining that the SoCal mountains were formed by earthquakes and if you travel the roads on the back way to Norton AFB from Yucaipa through Mentone through Waterman Canyon on Greenspot Road where you have the Mill Creek fault which merges into the San Andreas Fault, you can actually view the different layers of rock of the earth as they rose out of the ground.
Been looking for the Earthquake handout they gave us that night. Did find my Earthquake book that shows all the faults in the Yucaipa area. It talks about Mill Creek Canyon and the Mill Creek fault that eventually joins in with the San Andreas Fault in Waterman Canyon. Mill Creek Fault in the Yucaipa area is near the the San Andreas and was a worry to the geologist that night because if has a large enough earthquake, it could rupture the San Andreas in that area. It also talks about the San Jacinto Fault that parallels the San Andreas.
Of those 386 quakes I believe only one or two are M4; most are M 1-2 microquakes.
Each jump up the Richter scale is 32 times more strain energy released.
Those quakes aren't relieving much stress at all.
Main thing about small/medium quakes is that they each individual one really doesn't have any grand higher meaning; however, it's not a sign that that area is safe from a larger quake or anything. There are many situations (foreshock swarms, the Mogi Donut) etc. where they are in fact an indication of a larger quake to come.
Another good example is the SF Bay area from 1850 or so to 1906; you saw an increasing number of medium earthquakes (M5s, M6s) in the whole Bay Area, till the big shebang finally went in 1906....that quake actually DID relieve stress so quake activity in the area was very low till the 1980s; now it's increasing again.
Yucaipa.
Sorry, I really should include that when I make these images.
"The Banning segment of the San Andreas fault in southern California is associated with an average rate of background seismicity. This seismicity is scattered throughout the region and does not simply delineate the major strands of the late Quaternary faults. Some of this seismicity is caused by distributed deformation across the region because the southern San Andreas changes directions from northwest to the south of Banning to west-northwest near Cajon Pass."
http://www.cisn.org/special/evt.05.06.16/
Thanks. I never did take geology in college (Botany, Chemistry and Physics). I was thinking that the difference between each point on the scale was a factor of ten, not 32.
It's kind of fun to pretend to understand which cog or stress point will break next, and we can make some guesses based on what little history we really have of the process.
I think we do understand it enough to know where the major faults are, and the biggest fault line is the San Andreas running from east of Los Angeles to a point somewhat north of San Francisco. No news to anyone here.
It's going to keep chugging along and breaking as long as any of us are alive, and for quite awhile afterward.
Each jump is 10 times in terms of GROUND motion; but the total energy release of each whole number jump upwards is about 32 times.
That depends.... Did it move for you?
Also why there are few brick homes in LA. I miss brick buildings (including very large ones) from my roots in St. Louis.
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