April 2013 Oklahoma Earthquakes

This morning Oklahoma experienced another small sequence of earthquakes.  (There was also a large earthquake with an estimated magnitude of 7.8 in Iran over night.)  While I'm preparing for a conference very soon I'm a bit crunched for time, but thought a short post would be in order.  I would like to write a few posts concerning what magnitude is, how we calculate it, and other common questions I get asked at some point in the near future.

Okay, here's the synopsis of the most recent events. Early this morning at 01:56:29.875 CDT a magnitude 4.7 earthquake occurred centered northeast of the Oklahoma City metro area.  There have been a few significant aftershocks at magnitude 3.0, 3.6, and 4.6.  It is notable that there was a higher number of seismic events (though all small) beginning yesterday.  All these numbers are from the Oklahoma Geological Survey, the USGS estimates are lower with the largest events at 4.3, 4.2, and 3.3.  These magnitudes are computed on slightly different scales, but either way the largest earthquake released over 10 times LESS energy than the earthquakes last year.

The USGS did you feel it program has already collected around 1600 responses and the shaking reported matches very well with what was expected, probably due to the DYFI scale being pretty accurately calibrated after the large earthquake sequence last year.  It was striking that the vast majority of the responses came within 90 minutes of the quakes indicating the people actually got up and reported as soon as the event was over.  These responses really help the folks at the national earthquake information center (NEIC) and if you felt the earthquake but didn't go fill one out you should!

The moment tensor solution of the earthquake shows a strike slip solution meaning that the rock moved laterally past each other, not up and down.  This is shown by the "beachball" below with the colored regions indicating areas of compression.  There isn't enough information from one earthquake alone to tell if the fault runs SW to NE or SE to NW, but based upon the distribution of the large aftershocks it would be an okay initial guess that the fault trends to the NW.  Also notice the solution isn't perfectly strike-slip.  There is a small amount of oblique motion with a thrusting sense.  
After inspecting the infrasound instrument I have in my office I didn't see the earthquake, but the ground motion wasn't really detectable on the seismic station in Standing Stone, PA either.  It looks like the infrasound may have recorded the Iran earthquake, but I need to move it to a less noisy location.
Just for fun I've thrown in a seismograph below from a station in the Wichita Mountains in SW Oklahoma.  It would be fun to calculate the different arrivals and plot, but that's more fun for another time! I've made the trimmed .SAC file is available here in case you want to download it and try.

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