Mind your P's and S's

A pretty impressive M4.3 earthquake hit Medford Oklahoma at about 05:00 UTC today (160km N of Oklahoma City).   It shows up as a massive spike on the MAOK station

M4.3 event about 160km N of Oklahoma City recorded on 

station MAOK (upper) and our geophone site KEYWB(lower) 

On first glance it looks as though this event was missed my our geophone... however the MAOK station clock is running 5 minutes fast so we need to look at the wiggles occurring at 05:00UTC ...just when our geophone  trace skips from one line to the next (in jamaseis, click and drag the bit you want and click on Extract Selection) 

Zooming in on the time around 05:00 our geophone 

shows an impressive double-peaked signal 

The double peak is because earthquakes create two types of waves ... Primary P waves which are longitudinal waves (like sound waves in air) and Secondary S waves which are transverse waves (with a side-side particle motion).  The P waves travel a bit faster than the S waves so there is a time delay between the two sets of waves arriving

Because we know how fast P and S waves travel through the earth we can estimate how far away the event is by lining up P and S wave arrial times with the calculated travel times for P and S waves on a graph (in jamaseis you can do this by clicking the calculate distance tab in the zoomed data window) 

Graph of calculated P and S wave travel times vs distance with our seismogram overlaid, the seismogram is slid around on teh graph until the P and S waves line up with the curves and the distannce can be read off the Y axis.

 This gives us an estimated distance to the event of about 1.5 degrees.  (as every good sailor will tell you 1 degree = 60 nautical miles = approx 111 km (the circumference of the earth/360).)  So about 160km away !