Earthquake!

Yesterday at 9:56:06 Pacific time there was a magnitude 4.7 earthquake near Anza, California.  There were also a number of smaller quakes both before and after the big one.  I was outside walking our dogs when the shaking hit our house about 15 seconds later, and I never felt a thing (see seismograph below, from the seismometer in our house).  I did hear a neighbor's dog start howling mournfully at exactly the time the quake hit us, no doubt reacting to it.  The epicenter is about 77 km (44 miles) from our house, implying a seismic wave propagation speed of around 5 km/second, as expected.

Debbie was inside our house at the time, and she reports audible rumbling just before the main shock hit.  That is exactly what the seismograph shows, with smaller accelerations for about 10 seconds before the “big” secondary wave hit us at 9:56:31 (note that the seismograph is in UTC time, 7 hours later than Pacific time).

Earthquake!

Well, yesterday, a little teensy one, and quite a long way away.  What makes this one interesting is that the seismometer in our house captured it (see below).

USGS says that the earthquake occurred on 2/23 at 04:40:14 UTC.  Our seismometer shows the first burst of primary wave at 04:40:33 – 19 seconds later.  The location of the earthquake (USGS) is 109 kilometers from our house, yielding an average primary wave speed of 5.75 km/s, well within norms for primary waves.  By collecting lots of data like this, for many earthquakes and from many locations, USGS is building up a huge dataset that will let them learn a great deal about the composition of the earth's crust in the areas where they have good coverage (Southern California is one of them)...

Earthquake Captured!

Early this morning (2:36:02 Pacific time, 10:36:02 UTC) there was a fairly large earthquake (6.3 magnitude) off the coast of northern Mexico.  The epicenter was 305 km (about 183 miles) SW of our home.  At that distance, of course we never felt a thing.  But...the USGS requested data from the seismograph they installed in our home, and the graph at right shows what it “saw”.

This is the first seismogram I've ever actually tried to read.  I found several articles about reading them; this was the most useful.  If I'm understanding this correctly, I'm seeing the P-waves reaching us at about 10:36:46 UTC, or 44 seconds after the quake actually occurred.  That works out to a speed of 305/44, or 6.93 km/s – fast for a P-wave.  Then I'm seeing the S-waves reaching us at about 10:37:17 UTC, or 75 seconds after the quake actually occurred.  That works out to a speed of 4.07 km/s – again fast for an S-wave.  From what I've been reading, these seismograms are very challenging to extract good information from – both the amplitude and the timing of them are highly dependent on the geologic structures and material types between the earthquake and the seismometer.  I think I'll leave that to the experts :-)

But I do think it's kind of cool to be able to see the earthquake on an instrument in my house!