There has been a Heck Of A' Lot 'O Seismic Activity in Southern California lately! These events are relatively low in Magnitude ("MI") which are typically of 4.2 to 4.8, but there are so many events occurring in the same locations...
To me, it appears as if the Large Faults - such as the San Andreas (Fault boundary between the North American Plate and the Pacific Plate), are being relieved of tension in multiple small "Steps" by releasing stresses in many small 4.2 MI to 5.0 MI Events; rather than releasing the stresses in several very large events (> 7.5 MI)
These could also be fore-shocks... who knows!
The Events nearest to Us (Wife and Me) are on the Chino Fault Line, epicenter apx. 2 Miles from Yorba Linda. Most recent was on August 29th 2012 at 12:31 PDT. Event measured 4.1 MI, at a depth of 8.0 kM, at coordinates 33.907N 117.779W. Closest to epicenter: 3 km (2 miles) NNE (22 degrees) of Yorba Linda, CA.
As with the previous Events, when the P&S Waves arrived, there was an initial hard lateral thrust (like having a rug pulled out under your feet), rapidly subsiding down with "Back And Forth" lateral movements - lasting no more than 5 seconds, then ending with circular rotations for max. 30 seconds.
The initial "Jolts" scared the @#!*&% out of me! Each time, I was in "Full Engineer Mode" (extreme concentration on a given design element), and the room was dead silent!
Here is a list of the Seismic Events which have occurred since April 28 2012:
28 Apr 2012 07:07:27 4.1 MI 34.243N 117.426W Depth 10 km 3 km (2 miles) NW (316 degrees) of Devore, CA
28 Apr 2012 13:25:07 4.1 MI 19 km (12 miles) SW (236 degrees) of Guadalupe Victoria, Baja California, Mexico
22 May 2012 07:06:24 4.4 MI 14 km (9 miles) S (190 degrees) of Seeley, CA
29 May 2012 21:14:00 4.1 MI 48 km (30 miles) SSW (212 degrees) of Malibu, CA (Offshore between coast and channel islands)
10 Jun 2012 14:45:43 4.0 MI 64 km (40 miles) WSW (251 degrees) of Guadalupe Victoria, Baja California, Mexico
13 Jun 2012 19:17:15 4.0 MI 33.912N 117.783W Depth: 5.0 km 3 km (2 miles) N (11 degrees) of Yorba Linda, CA
30 Jun 2012 19:25:20 4.8 MI 21 km (13 miles) WSW (250 degrees) of Guadalupe Victoria, Baja California, Mexico
30 Jun 2012 19:25:23 4.6 Mb 16 km (10 miles) NW (306 degrees) of Guadalupe Victoria, Baja California, Mexico
7 Aug 2012 22:23:34 4.4 MI 2 km (2 miles) NNE (19 degrees) of Yorba Linda, CA
8 Aug 2012 08:33:22 4.5 MI 1 km (1 miles) NNW (348 degrees) of Yorba Linda, CA
--- And The Craziest Of Them All: > 24 Hours of Seismic Activity at One General Location!!! ---
Between 26 Aug 2012 11:16:11, and 27 Aug 2012 18:01:20, there were (18) separate Events, from 4.0 MI to 5.4 MI, in the Salton Sea area, nominally 4 km (2 miles) NW (326 degrees) of Brawley, CA. This location is the San Andreas Valley, and the Fault Line runs against the foothills located to the northeast.
That's all from Shakeytown!
-- Scott (EE)
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!
I know that the panic was hilarious to Californians, but keep in mind that we have many colonial era buildings in this area. There was millions of dollars in damage done to the National Cathedral and the Washington Monument, both over 75 miles away from the epicenter. That damage is still being repaired over a year after the event.
In towns closer to the epicenter, there was much more severe damage. In Madison, VA, the town lost its entire high school. Yep, they had to condemn it and tear it down and it wasn't that old. Other buildings had chimney collapses, etc. 200 year old houses just can't tolerate much vibration whatsoever!
As much as I admire all the effort put into 'earthquake' design, I think we're overly pessismistic.
Let me put it in very simple terms: I've been in quite a few 'normal' or 'old' buildings that are regularly shaken pretty severely by such things as passing trains.
One such place is my house. Nearly as old as I am, the house hasn't settled, tilted, twisted, or cracked a bit- even though you definitely feel it when a train couples itself to another car. Going by the subjective descriptions you can find, I'd guess the 'train shake' is 5.5-6.0 on the Richter scale.
As for the 'colonial' buildings .... well, if it wasn't for the "preservationists," most of the blessed things would have been knocked down and replaced by modern structures decades ago. IMO, the preservationists ought to be sent the repair bills and injury claims.
Blytheville, Arkansas. One can make the case that the Mississippi River has us surrounded; 'natural' river on three sides, and diversion canals on the fourth. Missisippi River ... looks almost like you can walk across it these days. Dead center over the epicenters of the New Madrid quakes. How's that for a seismic history? Tornadoes care born here. Hurricanes come here to die. Mostly of boredom. I have Isaac coming for dinner tonight. What? Me worry? Never!
Hurricanes are a coastal thing but the little tornadoes they spin off can come bite you. These are not the F3s and 4s you see in the midwest, they are really about what you see in the eye wall of a hurricane. It does test the uplift strapping on a building tho.
I know "what is uplift, isn't the force always down"?
Earthquake is actually a different force, it is sideways with a little up and down. I understand some of the mitigation is the same tho. We still have to deal with wind loads on walls.
I understand they are finally looking at wind codes up north. My builder nephew in law was telling me they are strapping trusses now in Maryland.