Tuesday, March 12, 2013


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).

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