4.5 Earthquake San Bernardino California 7:49 pm (Update LA Times is Reporting Quake was on Offshoot of the San Andreas Fault.)

A moderate earthquake has struck Southern California the USGS rates it at 5.0 at 9 miles below the surface near San Bernardino California.

A foreshock is a smaller quake that precedes a larger quake.  The largest quake in a series is considered the main one.  That's likely why the quakes on the map start in the red danger color as the threat of a quake being just a foreshock to a larger one recedes as time goes by.

The foreshock info is great for scaring (or is that scarring?) your kids after minor rumblers happen.  Well, if I didn't do the news people were going to, so......

Here 's a link to the USGS list of recent quakes in California and Nevada.  You lose the map but you can get more info from it.  I don't think it's the same one my wife found since it reports the depth of quakes in kilometers instead of miles.  Refresh the page to keep up with shocks as they happen

It was first reported as 5.0, and TV news is still calling it 4.9, but the USGS has reduced the strength to 4.5 on the Richter scale, a strange math construction where adding 1 multiplies the strength indicated by 10 times.  I think it was last used by Treasury Secretary Hank Paulson to determine what size the Wall Street bailout should be.

Update

LA Times reports in "Magnitude 4.5 quake felt across Southern California" that San Bernardino is reporting some minor damage. Also the USGS spokeswoman remembers the quake history of the LA Basin, and So. California to be the same as I do.

Excerpt:

U.S. Geological Survey seismologist Lucy Jones said the quake appeared to strike near the northern end of the San Jacinto fault, which is considered part of the San Andreas fault system. It was initially reported as magnitude 5.0 but was soon downgraded.

The temblor was comparable to the 1988 Pasadena earthquake and about 25 times smaller than the Chino Hills quake last summer.

"It's the type of earthquake we used to have in L.A. every year in the '80s," Jones said. "After Northridge, it went quiet."

Checking maps, I found that San Bernardino is close to the huge San Andreas fault  and the quake struck on one of the faults in the system, but not the big daddy itself, which is shown in red on the map clip above).  Aftershocks hit near San Bernardino and near Colton which is SSE of SB and looks like it is on the San Jacinto fault too. The shocks near Colton were at the same depth as the SB quake and aftershocks. 

Interestingly, earlier today there small quakes were felt near the lower end of the San Andreas at Niland, CA and in Idyllwild a small town on Mt San Jacinto. 

The San Jacinto fault appears to run South West of the mountain with the same name, but a map I saw earlier but can't find again shows a branch coming off the SJ fault called the Banning branch which runs north that might pass under Mt. San Jacinto.  The map didn't have enough detail to tell. 

Besides multiple quakes at SB and Colton there was a quake at Running Springs about the same time which is just NE of the San Andreas in the same ares, but not on the San Jacinto fault.

You can check out another map of the San Jacinto fault itself .  I don't see the Banning branch showing on it though.

Still, though the larger quake and several of the others were clustered around the SA Fault system most of them were too small even to worry about.

tags: san andreas fault  southern  california  earthquake  

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