Earthquake!

Earthquake!  Well, a little teensy one – just magnitude 2.2, yesterday at 8:24 am local time.  We didn't feel a thing here, and probably wouldn't have even if we were at the epicenter.  The seismograph of it at right (click to embiggen) was captured by the USGS seismometer bolted to our home's slab, about 4 feet behind me as I write this.  The epicenter was about 42 km (25 miles) from our house.  The earthquake occurred at 16:23:57 UTC, and the onset of the primary wave here was 16:24:05 UTC, just 8 seconds later.  That works out to a primary wave speed of 5.25 km/second (or 11,340 MPH), which is about the right speed for primary wave propagation through a very hard material like granite (which our local mountains are made of)...