Monday, March 5, 2012

Two at the same time? UPDATE

Okay, thanks, I was already awake.

About ten minutes ago there was a quake I felt here in Oakland.  There was that little start like someone unloading something heavy from the elevator down the hall, and then there were some serious left to right shakes.  After about three, I decided to stand in a doorway.  It was over soon enough.

Of course, I googled "seismic activity" and up came a 4.3 centered in Richmond, about 13 miles north of my house.  That was about five minutes after the shock, but a few minutes later, there was a revision.  There were two quakes, one in Richmond and one in Tiburon, over in Marin County, and both were measured at 4.0.

A little math help.  Two 4.0 quakes do NOT add up to an 8.0 quake, thank Odin, Vishnu and the little baby Jebus. The Richter scale is logarithmic and twice as strong only increases the reading by the log of 2, which is about 0.30103... That's why two 4.0 quakes would read the same as one 4.3 quake.

I don't ever remember two noticeable quakes centered so close to one another so close to simultaneous, though I will admit I don't keep close records.  With as many faults and fissures as there are in the Bay Area, I'm a little surprised that movement in one fault line wouldn't nudge another into a quake more often than this, if that is in fact what happened.

UPDATE: In the comments, Susan S. said it had been changed to one quake, so I re-Google seismic activity and as of noon, it's two quakes, eight seconds apart, the first one 3.5 and the second 4.0. Both are centered in Richmond, so not one fault triggering another.


susan s. said...

That has been revised to _one_ quake since about 9am.

Matty Boy said...

Googling "seismic activity", now it says a 3.5 followed by a 4.0 about 8 seconds later, both in Richmond. Tiburon is no longer in the picture.

Noon on Monday.

namastenancy said...

Well, one quake or two, it sure scared the heck out of me. I was really deeply asleep and was totally disoriented and couldn't go back to sleep for the longest time.